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include/constants/opponents.h, src/data/trainer_parties.h and src/data/trainers.h, I believe.
There's a total of 855 trainers in the game by default that you can repurpose, and 9 more slots to add entirely new trainers in for a total of 864 trainers.
You should focus on using up all those slots before you try to expand the amount of trainers you can add.