The original Pokémon generation contained 151 Pokémon.
Pokémon Red and Green introduced the first Pokédex, an in-game encyclopedia designed to record the species encountered and caught by the player. The numbered collection ran from Bulbasaur through Mew, although Mew was hidden and was not normally obtainable through ordinary gameplay in the original cartridges.
Many players remember 150 because Mewtwo is the final commonly encountered Pokémon in the main story and because Mew was distributed separately. The complete first-generation total is 151, with Mew counted as the final species.
The number later expanded with every new generation. Pokémon Gold and Silver added 100 more species, bringing the National Pokédex total to 251. The original 151 nevertheless remain especially important because they defined the characters, types, and designs that introduced Pokémon to the world.