An unbounded number of simulacra (provided you have access to enough diamonds, or the wish spell)
The adventure
Storm King's Thunder
has a modified version of the Simulacrum spell
Found in Sansuri's enormous spellbook, in the Library (room 21 of Lyn Armaal) containing the spell Sansuri's simulacrum (7th level) (emphasis mine)
This modified version of the spell has the following modifications:
- It allows multiple duplicates of the same creature to be created
- Requires a powdered diamond worth 5,000gp as an additional material component
- Both the powdered diamond and the "regular" 1,500gp powdered ruby from the regular version of the spell must be sprinkled over the duplicate, and are both consumed by the spell
Otherwise the spell is as the spell simulacrum in the PHB (7th level)
This modified version very much breaks the balance of the spell...lets melt some cheese.
Basic breaking of the cheese
If this modified sumulacrum spell is paired with the spell wish, then you can use the basic use of the wish spell to:
duplicate any other spell of 8th level or lower. You don't need to meet any requirements in that spell, including costly components. The spell simply takes effect.
So, assuming you are at least a 17th level spellcaster, the procedure is:
- Cast the modified version of Simulacrum on yourself using a 7th level spell slot
- Take a long rest should you wish to regain your 7th level spell slot
- Have your Simulacrum cast wish to duplicate the modified Simulacrum spell on you
- Have your Simulacrum command each new simulacrum to obey all of your spoken commands as if it were their own
- Command the new Simulacrum to repeat the use of wish on you duplicating this modified version of the spell.
- Repeat
Et voila, you have an unbounded number of Simulacra, who are only missing their 9th level spell slot.
This modified spell avoids any pesky side cases whereby your simulacrum would be destroyed by the simulacrum casting wish to duplicate simulacrum.
But my DM (set of rules) doesn't allow Simuacra to cast or duplicate Simulacrum - more RAW friendly cheese
Alternatively, if your DM pulls out some pesky rule like "simulacra can't cast or duplicate the simulacrum spell in any way":
- Cast the modified version of Simulacrum on yourself using a 7th level spell slot
- Cast the modified version of Simulacrum on yourself using a 8th level spell slot
- Cast the wish spell to duplicate the modified version of Simulacrum on yourself
- Take a long rest to regain your spell slots
- Repeat
Now, so long as you have a plentiful supply of 5,000 gp diamonds and 1,500gp rubies, you can have an unbounded number of Simulacra with the following properties:
- 1/3rd will have no 7th level slot, but will have 8th & 9th level slots (with wish prepared)
- 1/3rd will have no 7th & 8th level slots, but will have their 9th level slot (with wish prepared)
- 1/3rd will have no 7th, 8th & 9th level slots
I don't have diamonds but I do have wish! wish based cheese
Finally, if you have no access to a plentiful supply of diamonds and rubies, but you do have access to wish, the procedure is:
- Cast the wish spell to duplicate the modified version of Simulacrum on yourself
- Take a long rest to regain your spell slots
- Repeat
You now have an unbounded number of simulacra of yourself possible, missing only their 9th level spell slot. You just need to figure out a good way to make yourself live longer to be able to make enough Simulacra to try to take over the world.
How can I live long enough to make an unbounded number of simulacra? cheese that aged so well it's reaching through the editions
Possible solutions are the Clone spell, sufficient Potions of Longevity, or (the most efficient way imo), access to the Ioulaum's Longevity spell from the 3.5e Lost Empires of Faerun sourcebook (or listed, but not described, in the Netheril Empire of Magic boxed set from 2e) which enables you to:
[deal] 20d6 damage to every creature within [a 120ft radius]. A living creature that is reduced to -10 or fewer hitpoints (or construct or undead that is reduced to 0 hitpoints) is utterly destroyed as if disintegrated, leaving behind only a trace of fine dust. This spell also extends the caster's current age category by 1 year for every living creature it slays.
The spell's duration is instantaneous, but the age extension is permanent
If you have access to this spell, and the wish spell then you can functionally create an unbounded number of Simulacra by using the following methodology to increase your age by 365 years once every century or so:
Since your Simulacra are creatures, with half of your hitpoints, once you have a sufficient number, order 365 of them to beat each other with their fists until they have 10 or so hp (this should be easy to judge, given they are all you), then order those 365 to pile themselves into a 120ft radius circle, cast Ioulaum's Longevity. Congratulations, you have just increased your age by 365 years, at the cost of 1 years worth of Simulacra. Create as many Simulacra as you want for 364 years and repeat this method to live forever and have an unbounded number of Simulacra.