Finding out how many Pokémon are in Pokémon GO is a difficult task due to so many of them being specific to certain regions of the world or exclusive to events. Pokémon GO, surprisingly, hosts nearly all the Pokémon from every generation of the main series, other than some from the more recent generations and a few Mega Pokémon evolutions. Over the past couple of years, Niantic has introduced several hundred Pokémon into the app, nearly completing the Pokédex at one point.

https://screenrant.com/pokemon-go-pokedex-missing-not-included/