Sansuri's Spell Modding Simulacrum Mayhem -- or infinite simulacra, 1/cast (Warning: May be game or world breaking. Consult DM and/or your favorite deity of magic before casting.)
The text of the base simulacrum spell in D&D 5e reads as follows (italics mine):
You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell. The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature. It appears to be the same as the original, but it has half the creature's hit point maximum and is formed without any equipment. Otherwise, the illusion uses all the statistics of the creature it duplicates.
The simulacrum is friendly to you and creatures you designate. It obeys your spoken commands, moving and acting in accordance with your wishes and acting on your turn in combat. The simulacrum lacks the ability to learn or become more powerful, so it never increases its level or other abilities, nor can it regain expended spell slots.
If the simulacrum is damaged, you can repair it in an alchemical laboratory, using rare herbs and minerals worth 100 gp per hit point it regains. The simulacrum lasts until it drops to 0 hit points, at which point it reverts to snow and melts instantly.
If you cast this spell again, any currently active duplicates you created with this spell are instantly destroyed.
The italicized line was introduced intentionally over prior editions (the text of the spell in D&D 3.5e lacks any equivalent to the last paragraph expressly forbidding multiple simulacra, but is otherwise functionally identical) -- it is intended as a designer constraint on the power of this spell. The simplest way for the spell to be modified to lift this limitation would be to remove it altogether, rendering it equivalent to the behavior of previous editions, which allowed multiple simulacra to be active simultaneously. (Note to 3.5e geeks -- I am sweeping XP costs under the rug here as insignificant intentionally to avoid clouding the argument.)
Note also that simulacrum lacks any text describing what it does when cast out of a higher level slot. This means that the only way to accomplish the modification would be to lift this restriction in the spell text -- any numerically-based easing of the restriction would have to be keyed off of the spell slot level, and there simply aren't enough spell slot levels left in the game to accommodate the 18 aarakocra simulacra that Sansuri spammed Lyn Armaal with (even a square-law progression of simulacra/casting, with the restriction otherwise in place, would leave Sansuri with only half the simulacra she needs).
Note, of course, that lifting this restriction from the spell text doesn't mean that Sansuri's modified spell can create multiple simulacra/cast -- that would require much more drastic alterations to the spell text.