The first generation of Pokémon introduced 151 species.
These original species run from Bulbasaur, numbered 001 in the National Pokédex, to Mew, numbered 151. The first generation appeared in the Japanese Red and Green games and later in the international Red and Blue releases, with Pokémon Yellow adding a revised version of the same original set.
A common mix-up is saying that the first games contained only 150 Pokémon. The number 150 refers to the main obtainable species before Mew is counted. Mew was hidden in the games and was initially distributed through special Nintendo events, but it is still the 151st Pokémon in the National Pokédex.
The first-generation roster established many of the franchise’s most recognizable creatures, including Pikachu, Charizard, Gengar, Eevee, and the three starter lines. Later generations expanded the Pokédex by adding new regional groups rather than replacing the original 151.