No because a half HP Simulacrum of a high CR creature is lower CR than it.
If DM assumes CR = level, then, at first glance, this works.
But the DM should realize that letting this works will open a big can of worms onto the campaign.
Because the player could then ask his obedient copy to cast Simulacrum + True Polymorph to create a 2nd copy, and then the player would order his 1st copy to tell its copy to be obedient to you. And so on and so on, each new copy making another copy, ad infinitum, giving the player gains access to nigh-infinite power through an army of 100% loyal high level spellcasters.
Personally, as a DM in front of a player trying to do this, I basically see a form of immortality stunt: "I'll just send my simulacra on that dangerous mission in my place that way I can never die!" or even worse: "Hey wait I won't get any adventure experience that way, so nah I'll just have my copy accompany me on all my adventures!". The campaign balance would be solidly thrown out of whack! So I see trying such a "combo move straight from campaign hell" as basically a dick move which means to me the player just put on a white shirt with a nice big fat juicy red target painted on it saying "I'm a jerk, now punish me!".
For comparison purposes, a DM would not allow his fighter PC player to just say: "Hey I just so happens to have a twin brother that is as good of a fighter as me, he's very loyal to me, but has only half my HP, one less feat, and he doesn't have any equipment, but I give him armor and weapons and he'll come help me fight in the dungeons!". We all knows that that would screw up with all the further challenges down the line.
To limit this kind of exploit, the DM has several methods at his disposal:
Method 0: Allow it because it is a solo game.
If the party definitely lacks members to be able to face off its challenges, this can be a good player-provided way to "add a helpful NPC" into the group. Just don't suggest it yourself: A 100%-obedient-puppet NPC is not as fun for group roleplay dynamics as a DM-controlled one that is able to demand his own share of the loot and refuse insane requests.
The goal of the game is to have fun, after all.
But even in a 2 or 3 players game, the DM should be quite hesitant. Because as a DM, he needs to make sure that one player can't completely overshadow everybody else! Oh wait, he's a high level spellcaster. Of course that means he's already overshadowing every non-spellcaster around by a thousand miles. It is up to the DM to determine if this will be a positive or a negative, then.
But still, warning bells should be rung all around. But in a solo game not linked to a normal campaign, especially if it is a one-shot game, then allowing this is quite ok. But if the DM is allowing this in solo-play while the same PC plays in a group in a normal campaign, the DM should reduce the total loot and XP to what a solo adventure should really deserve, otherwise that PC will end up way too easily pulling ahead of the rest of group.
Even if it is allowed in a group, the DM should adjust XP accordingly: while a Simulacra can't earn XP, that doesn't mean that it doesn't get to deserve it's share: it just means that that share of the XPs that would go to that creature gets wasted away. More "NPCs" means less XPs for the players. The DM should also make sure that the party puts in the adventure only the normal loot that they should have for their normal number of PCs "as if" they were facing only party-size-and-level appropriate challenges, not the full loot for those monsters. Otherwise, their new "non-loot-taking" slave would allow them to fight stronger foes, allowing them to pull ahead of the level-power-curved too easily. But at that high-level quite late point in the campaign, the PCs are probably already swimming in loot anyway, so maybe it is not a problem.
Method 1: Copy has 1 less unused spell slots than creator.
Jeremy Crawford tweeted an answer to this aspect: A simulacrum is a
duplicate of another creature at the time of the spell's casting. You
essentially take a snapshot of that creature's game statistics at the
completion of the 12-hour casting, and those become the statistics of
This is 100% supported by the rules, albeit quite minor in limiting this exploit.
The Simulacrum spell does not indicate that it replenishes spell slots.
The new copy is created by the spell, so you must cast the spell first. So every new copy down the line will have one level 7 spell slot less than its predecessor. If you are level 20, this means a grand total of 4 Simulacrums of yourself, maximum.
But even that would already be giving a lot of "free" power, just for the cost of a single level 7 spell slot cast before the dungeon, the player gets effectively at least double the normal amount spellcasting actions in fights. Insane.
Method 2: DM interprets "you" in the most general sense.
This is a DM interpretation, not strict 100% rules RAW reading.
If you cast this spell again, any currently active duplicates you
created with this spell are instantly destroyed.
The DM simply says that a Simulacrum, despite being a separate creature with it's own "brain", is 100% obedient mainly because it does not really have it's own separate willpower, but is, in a way, some kind of extension of you. You are it's real source of will, if you might say so. Physically, the Simulacrum does the casting of its spells, but, for the purposes of determining the magical links and stuff, you're definitely in there, too. So for any spell with a "if you recast this spell, previous instance goes poof", anybody casting the spell in the "simulacrum chain" counts.
Side note: In fact, I personally find Simulacrum so abusive and powerful, that in such a situation, I'd even make all Concentration spells also need to be supported not only by the Simulacrum's Concentration, but also by the player's PC's Concentration at the same time. Even when the Simulacrum is a perfectly normal one of some random lower level spellcaster dude you copied. This would limit a lot the abuse potential of this spell. Sure yes, this is a stretching and bending of the rules, yes. But I wouldn't feel too guilty about it: after all the player just tried to stretch and bend the rules, too.
In any case the DM is allowed to change rules as he sees fit. The RAW-as-intent of the "no more than one instance to prevent abuses" part is pretty clear here.
Method 3: CR does not equal level.
This is 100% supported by the rules, and closes the exploit entirely.
The Simulacrum only has half HP. According to DMG monster creation rules, half HP of a high CR monster, definitely reduces the monster's CR to something lower than its raw HD. At least by 1, in any case! So it would not work at all.
If this is the player's "first offense at trying to be too power hungry", and an honest mistake, then this is the method I would use.
Method 4: The "punitive" approach
Sometimes you've got a player that can't help but repeatedly try to act like an abusive powergamer. You know the kind. They read the rules in minutiae, trying to find loopholes, always interpreting the rules their way. When it favors them? Very liberal interpretations. When it penalizes them? Ah there only the super-strict interpretation must be applied. And so on. You constantly enter in rules arguments with this guy and because he can't take no for an answer (even tough as DM you are perfectly in your right to do so) until he's pushed the issue as much as he can.
If it's this kind of player, that reads the rules in detail to find loopholes and exploits, then this approach here is the one I'd use. After all, he spends way more time reading the rules than me: he actually should knows the rules way better than me, at depth, yet he chose to completely ignore that the copy is lower CR than it's HD. He's got zero excuses here: he is definitely trying to pull a fast one. Merely saying "no" only means he'll try some stupid strunt again later, and again a long game-time-wasting argument while the "good" players just wait. Nah, this needs some punishment instead.
Because if he's willing to ignore the rules, to the point of starting to argue with me that he should be able to do it despite me saying "No, both because the rules don't allow it and because I don't allow it either!", then I can start ignoring and tweaking the rules, too. And hey, I'm the DM, I do have the right to change the rules anyway! Says so right on DMG page 4:
And as a referee, the DM interprets the rules and decides when to
abide by them and when to change them.
So, the player makes his "perfect" copy. Then I just make it so the casting of True Polymorph on the Simulacrum makes it "a little bit too real and too much like you": Despite being a copy, the creature is so virtually identical (except for having no equipment), that the new "you" ends up wanting to be the "you" that is really the "you" that is "you" instead of "you", and thinking that he is the real "you" for real. More than "you", in fact. If you catch my drift.
Your copy is the one thinking it should be the boss. Lots of funny mayhem might ensue.
Or it could happen like this:
"You wake up the next day and all your stuff is gone and you are not in your room. The spell must have worked wrong, because your simulacrum copy took all your stuff and even took your place! And he dares sending YOU, the original one, to do this suicidal mission! And he must have put a curse of a geas on you because you are somehow forced to obey! What do you do?"
The player will eventually guess that he is the copy, way before seeing that he can't level up (because he's a Simulacrum), and even sooner when he sees that he has only half hp and can't regain his spells. And he is forced to obey "the other one". Also sucks big time that all his best gear is on "the other one".
I'd allow the player a save to "break free" of the control. Which in that case would mean that something definitely went wrong with the spell: Either you got a freewill Simulacrum, or the soul of the PC did switch place with his Simulacrum and that is what allowed him to break free (which could allow regaining spells, too -- At that point, anything goes!).
Once the player confronts the DM, don't allow him to just Dispel the copy. Tell the player that if he dies on the "suicide mission that the Simulacrum (now the player!) is sent to do", then it's going to be "time to roll a new character", not "go back to playing the original 'true' PC". Then once he has played the Simulacrum well and long enough, he can get some kind of "appropriate end" for it (either killed by the original that doesn't need him anymore, killing the original, magically fusing the two together back into one, or any other story-satisfying arrangement).
Basically, in the best of cases the player ends up with some kind of "second life death protection", but the player also had to play for a while without his best gear on him. In the worst scenario, when he gets back his true PC, or the copy finds a magic to absorb the original into himself and thus become "truly alive" again, the player discovers that, while everybody else leveled up, he stayed the same level. Docking XP is a great way to say to a player: "Here, this is a slap on the hand, don't do it again". But turning it into an adventure can be a great fun way to resolve the situation. The player had his exploit work, "kinda". The other players can laugh about how the hyper-power-hungry player got hoisted on his own petard. Life goes on.
After all, simulacrums, even polymorphed, cannot gain XPs, and sending a simulacrum to take dangers in your place means you don't gain those XPs either.
But what if the player enters into a big argument?
Some player might argue that the True Polymorphed creature should retains the original Simulacrum's mindset, and thus remains loyal to the PC in all cases, even if the DM makes them play the copy instead of playing themselves.
But, seriously, allowing this combo, even if it means you can make even one single copy of yourself, at your full caster level, loyal to you, that inevitably ends up being way too much cheese headaches for the DM that way, and throwing a monkey wrench in the powergamer player's uber-power-abuse plans is par for the course. Still, in the face of a really abusive and argumentative player, there is another possible alternative:
Method 5: It works too well.
DM: "Ok, you got your tons upon tons of super-obedient super-powerful copies, all powerful spellcasters, and now essentially basically you are well on the way to move towards gaining ultimate magical power over everything, right? After all, that is your stated goal, to gain ever move power."
Player: "Definitely! So my next move is..."
DM: "Nah, no need: You inevitably succeed! After months of intensive magical research and experiments, your ultimate wish finally unfolds, and your PC and all of his copies make some kind of uber-wish-ritual to control even more magical power, and you become some kind of ultimate magical being. You have become free from being affected by the power of other ultimate powerful beings like you. You escape the confines of your limited mortal body and the rules of reality of this limited physical universe, and your unfettered newfound divine magical willpower simply moves out of this now too tiny Prime Material Plane prison and moves to the Grand Beyond of the real multiverse, in order to create your very own universes. Basically, you are now freed from me, the DM, and you can go become the "ultimate being" aka the DM of your own campaign: Goodbye! If you ever want back in my campaign, though, you'll start back at level 1... Personally I'm done with a player attitude that is way too power-hungry and argumentative, I prefer to focus on the roleplay than on never-ending rules discussions about the abuses of loopholes in the rules. So if you come back you'll start back at level 1, at least that way you'll have some room for huge improvements as you try to catch up to all the other high-level players, and I'll have less headaches dealing with that kind of attitude. Did I say goodbye? I guess it doesn't hurt to say it again: Goodbye!"
When a player just can't accept a DM's interpretation of the rules (see DMG page 4), then sometimes you have no choice but to kick him out out of the campaign. At least he gets out thinking he "won" the campaign.
Then you promptly make his PC's action never talked about as if they never happened. If the player comes back and gloats and talk about his previous PC:
"Nah, dude, that never happened in this campaign. It was all madness in the head of some crazy wizard that was so full of himself and so power hungry that he put all his efforts to twist and bend what magic could do until he ended up just killing himself. Also, new house rule, by the way: Players can note a rule to me ONCE, then I the DM make a decision and that's final. Keeping on arguing again means you auto-fail whatever you attempt in the worst way possible, plus your PC gets cursed with a nasty unremovable divine curse for at least one game session."
So yeah as PER RAW, consider "half max hp means CR < HD".
And don't ever let a player completely break the power balance in your campaign. Which, also as per RAW, is a big no-no, too.
So, DM's choices, short version:
"Copy appears after spell slot was spent to cast the spell, and Simulacrum doesn't replenish spell slots": Max Simulacrums is thus limited by level 7+ spell slots player has. Max 4 at level 20. 100% RAW Rules compatibility.
"The term 'You' includes the entire chain": Max Simulacrums is 1 no matter what. No "simulacrum chain". 95% RAW "Rules As Intent" compatibility.
"Half HP means CR < HD". It just plain doesn't work. <-- Recommended way! 100% RAW Rules Compatibility.
And then the "hoisting player up on his own petard" ways:
"Make the PC play the Simulacrum instead of his PC for a while".
"It works too well / Force-roll a new non-abusable PC / Kick the player out".
Those last two are not RAW rules supported. But a DM has the right to teach a player that trying to screw up with the campaign or with him can have real consequences.