The first-generation Pokédex included 151 Pokémon species.
These species debuted in Pokémon Red, Green, Blue, and Yellow, whose main setting was the Kanto region. The Pokédex numbering begins with Bulbasaur at #001 and ends with Mew at #151.
The first generation's count is often confused with the number available in a single game. Some species were exclusive to one version, while others required trading or evolution, so completing the full regional collection involved exchanging Pokémon between games.
Mew was especially unusual because it was not normally obtainable through ordinary gameplay in the original releases. It was nevertheless the 151st species in the National Pokédex, following Mewtwo at #150. Generation II later expanded the National Pokédex to 251 entries by adding 100 new species.