If someone had the Deck of Many things, drew the Moon and got a wish spell, and wished to "Level up as much as (They) can", how much XP would the wish spell give to the player?
Well there's no base rule on that, but there are several issues with the idea, mainly with how the DM decides the outcome of your wish.
If player tried to have their character say "I wish to be higher level", the DM would very likely immediately go "hold on, your character is not aware that they are in an RPG game with mechanics like that, you can't have them say that", or if they're really mean they'll just take that into consideration when they decide how they want the wish to come out. You might just end up several "building levels" up in the air and fall flat on your face.
So keeping to realistic ways your character would wish to become more powerful, you'd have to say something like "I want to be more powerful" or "I want to be stronger" which might give you better gear or increase your stats, but not your level.
Basically put, it's practically impossible to realistically word a wish in a way that will force the DM to only interpret it as giving you experience points.
This really falls outside the spell as it is written. It has a certain list of pretty failsafe results, and anything beyond that falls under DM purview. The guideline given is:
The DM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong.
The description then goes on to show how a DM can creatively interpret extremely powerful or game-altering wishes, usually to a degree less than desirable for the wisher. Most DMs would require you to word the wish in such a way as your character would be able to.* I.e., since your character has no concept of the terms "level", "XP", or "Ability Score" — those are game mechanic terms — the DM might require you to phrase it as "I wish to achieve my maximum potential in my skills and abilities, immediately!"
The the DM would (rightfully) figure out how to grant the letter of the wish without breaking the game you are playing. Most DMs and players would not relish playing in a party of five 4th level characters and one 20th level character.
A harsh or vindictive DM could simply strike you dead with a "bolt from the blue" at that moment, declaring that since you cannot now get any better, you have achieved your maximum potential. A very kind DM might find some middle ground or suggest you reword the wish.
* This is not an absolute. Some tables don't make much distinctions.
Recomendation: A single level
The other answers of 'your DM is most likely to be a dick if you ask that' is my single biggest issue with the magic in general, and not the answer to the question.
I have seen comments in previous places (Relating to 3.5 but still relevant in spirit: Can a character gain two levels with the same encounter?) suggesting that a single level is the most that should be granted by a single 'block' of experience, and that includes the card in the same deck which grants a specific amount of XP (IE: It effectively nerfs that card for the sake of gameplay).
If you stick to a single level you give the wisher something without causing too much imbalance. That said if you are playing with an item such as the Deck of Many Things you probably don't care about balance all that much, in which case it is something that has to be considered on a per-game basis.
If you are the DM you might want to think about how your campaign would cope with whatever difference in levels anything >1 would create, if you are not the GM you might simply want to ask. As a GM I would do everything in my power to grant the wish as worded, but asking for something game breaking would (Unsurprisingly) break the game and everyone would prefer to avoid that.
This said; the comments from the others on this site are still valid, word the Wish as the person in game would, and be prepared for your GM to be a dick.
Hmmm... "Level up as much as they can"... How many XP do they currently have? (including XP for the current adventure to date). What level does that entitle them to?
If that's more than their current level, Bamf they get to level up without having to go through any of the usual training.
Otherwise, they're already leveled up as much as they can at the moment :)
While not a duplicate, I would recommend looking at this thread.
It seems a common concept to players that a "wish" is a game-ender. You can suddenly get ultimate power with no work. Instead, it should be treated as a deus ex machina; a one-time boon to get out of a sticky situation.
In fact, the less the player asks for, the more likely a DM is going to grant it, and grant it in a way that matches what the player really wanted.
"I wish I had drawn 'The Sun' from this Deck."
As long as your character has some reason to know what the cards in the deck do, this is pretty simple. That's 50,000 XP. Probably your best shot of instantly leveling, or at least moving yourself along.
The next option would be If you want a level, and given that you had already drawn 'The Comet' you might wish to be attacked immediately by a very weak monster you can beat in one shot. It's unlikely to line up like this for you, but it would work, and if you are just exactly at the XP for Level 19, this would probably give you the absolute max for a Wish spell.
The other answers don't mention it, but it's also possible for a Wish to have no effect:
You might be able to achieve something beyond the scope of the above examples. State your wish to the DM as precisely as possible. The DM has great latitude in ruling what occurs in such an instance, the greater the wish, the greater the likelihood that something goes wrong. This spell might simply fail, the effect you desire might only be partly achieved, or you might suffer some unforeseen consequence as a result of how you worded the wish.