Highest number of Pokémon you used in a single playthrough

16 in HeartGold, because I basically made a team for Johto and then created a completely new team for Kanto. Would've been a smoother experience if the HG/SS level curve was better, though! :p

on a non nuzlocke playthrough i had around 18 in black, i like catching lots of pokémon so some early route ones get used; for annoyingly practical purposes some are utilized as hm slaves.

Probably during my first Shield playthrough when I used around 14 different Pokémon during the playthrough, at times I was rotating Pokémon pretty often with the Box Link. Levelling wasn't really a problem either due to the Exp Share and Exp Candies, so it was pretty easy to rotate my team in Shield.