The original Pokédex of the first-generation Pokémon games included 151 Pokémon.
These Pokémon correspond to the creatures introduced in Pokémon Red, Green, Blue, and Yellow. Bulbasaur was National Pokédex number 001, while Mew was number 151.
The original Japanese releases used Red and Green, with Blue arriving later as an updated version. International releases generally used Red and Blue, but the same first-generation roster formed the basis of the Pokédex.
A common mix-up is counting only the 150 Pokémon obtainable through ordinary gameplay. Mew was the 151st Pokémon, but it was distributed through special events rather than normally encountered in the games.