Generation I introduced 151 Pokémon species in Pokémon Red, Green, and Blue.
These games began the Pokémon series in Japan in 1996, with Pokémon Red and Green released for the original Game Boy. Pokémon Blue followed in Japan, while international versions were released as Pokémon Red and Blue. The original Pokédex ran from Bulbasaur at number 001 to Mew at number 151.
The number 151 is often confused with 150 because Mew was exceptionally difficult to obtain and was not normally available through ordinary gameplay in the original releases. Mewtwo is number 150, while Mew is the final species in the first-generation Pokédex. Later games added new generations, forms, and regional variants, but those do not change the number of original Generation I species.