The original Pokémon Red and Blue Pokédex included 151 species.
Those 151 Pokémon make up Generation I, the first group introduced in Pokémon Red, Green, Blue, and Yellow. The count begins with Bulbasaur and ends with Mew, although Mew was not normally obtainable through standard gameplay in the original releases.
The first-generation Pokédex includes the three starter families, version-exclusive species, fossil Pokémon, legendary Pokémon, and familiar creatures such as Pikachu and Eevee. Mewtwo is number 150, followed by Mew at number 151.
A frequent mix-up is saying the original games contained only 150 Pokémon because Mew was distributed through special events rather than ordinary in-game encounters. The official Pokédex still counted Mew, so 151 is the recognized total for the original generation.