The first generation of Pokémon introduced 151 species.
These original species appeared in the 1996 Game Boy games Pocket Monsters Red, Green, and Blue, later known internationally as Pokémon Red, Green, and Blue. They occupy National Pokédex numbers 001 through 151, beginning with Bulbasaur and ending with Mew.
Mew is an important source of confusion. Players could encounter 150 Pokémon through the ordinary game content, while Mew was a hidden mythical Pokémon that was distributed through special events and promotions. It was still included as National Pokédex number 151, so the complete first-generation total is 151.
Later generations expanded the Pokédex substantially, but fans often use “the original 151” to mean the species from the earliest games. Pokémon such as Pikachu, Charizard, Eevee, Mewtwo, and Mew all belong to this first-generation group.