The original Generation I games featured 151 Pokémon.
Pokémon Red, Green, Blue, and Yellow introduced the first generation of Pokémon. The Pokédex numbered the creatures from Bulbasaur at No. 001 through Mew at No. 151, creating the familiar goal of collecting them all.
The number can cause a small historical confusion. Many players completed the main in-game Pokédex after obtaining 150 Pokémon, because Mew was distributed separately and was not normally obtainable through ordinary gameplay. Mew nevertheless belongs to the original 151 and occupies the final National Pokédex position of Generation I.
Later generations steadily expanded the total. Generation II added 100 new Pokémon, bringing the running total to 251. The original 151 remain especially prominent because they were introduced before the worldwide Pokémon boom and formed the foundation for the anime, trading-card game, merchandise, and first film.