The first generation of main-series Pokémon games introduced 151 species.
Pokémon Red and Green launched in Japan in 1996 with a Pokédex that ran from Bulbasaur at number 001 to Mew at number 151. These games established the original collection of Pokémon associated with the Kanto region.
The figure includes Mew, even though Mew was not normally obtainable through regular gameplay. It was distributed through special events and promotions, which sometimes leads people to say that the original games had only 150 obtainable Pokémon.
The first generation also established the series’ core loop: catching Pokémon, training them, battling with them, and trading them with other players. Later releases added new species in generations, but the original 151 remain the foundation of the franchise’s 30-year history.