The original Pokédex introduced by Pokémon Red and Green contained 151 Pokémon.
These species make up what fans now call Generation I. The first 150 entries run from Bulbasaur through Mewtwo, while Mew is traditionally treated as the hidden 151st Pokémon. Players could encounter, obtain, or trade many of the species, but no single version contained every Pokémon without trading between games.
The number 151 is closely associated with the franchise's early identity. Later games expanded the Pokédex with Generation II and added 100 more species, bringing the total to 251. That later expansion is why older references to “all the Pokémon” can mean either the original 151 or the much larger modern roster.
The original Japanese games were Red and Green. International versions used Red and Blue, but they retained the same original-generation Pokédex count.