The original first generation contained 151 Pokémon.
These Pokémon debuted in Pokémon Red, Green, and Blue, with Mew counted as the 151st entry even though it was not normally obtainable through standard gameplay. The first-generation Pokédex therefore runs from Bulbasaur at number 001 to Mew at number 151.
The number 151 is associated with the Kanto Pokédex, whose creatures formed the foundation of the franchise’s collecting concept. The original games distributed different Pokémon between versions, encouraging players to trade through the Game Boy Link Cable to complete their collection.
Some players remember the total as 150 because Mew was hidden and distributed separately from the main adventure. Others count only Pokémon obtainable in ordinary gameplay. The canonical first-generation total is still 151, and later games expanded the overall National Pokédex far beyond that original group.