The original Pokédex from the first Pokémon games listed 151 Pokémon species.
Pokémon Red and Green introduced the first generation of creatures, numbered from Bulbasaur at 001 through Mew at 151. The total included Mew, even though it was hidden and was not normally obtainable through standard gameplay in the original releases.
A common mix-up is saying that the first games contained only 150 Pokémon. That number reflects the traditionally catchable species in the original games, while Mew was the additional Mythical Pokémon associated with the complete first-generation Pokédex.
The next generation expanded the Pokédex to 251 species with Pokémon Gold and Silver. Later games continued adding new regions, mechanics, and Pokémon, but the original 151 remain the foundation of the franchise’s creature roster.