The original Generation I Pokédex listed 151 Pokémon.
These species debuted in the first-generation games associated with the Kanto region. The sequence began with Bulbasaur at number 001 and ended with Mew at number 151, although Mew was difficult to obtain through ordinary gameplay in the original releases.
The total is often confused with the number available in a particular game version. Pokémon Red and Blue divided some species between versions, encouraging players to trade. Pokémon Yellow also used the same first-generation roster but changed parts of the adventure and starter setup.
Later generations expanded the Pokédex substantially. Generation II added 100 new species, bringing the cumulative total to 251. Because the franchise now contains many more Pokémon, the number 151 specifically refers to the original first-generation set, not the complete modern Pokédex.