Pokémon Red and Blue originally featured 151 collectible Pokémon.
Those 151 species make up what fans commonly call Generation I. The games challenged players to obtain Pokémon through catching, evolution, in-game gifts, and trading between versions. Because some species were exclusive to one version, completing the Pokédex required exchanging Pokémon with another player.
The first 150 entries ended with Mewtwo, while the Mythical Pokémon Mew was numbered 151. Mew was hidden in the original game data and was distributed through special Nintendo events rather than normal gameplay at launch.
Players sometimes remember the original total as 150 because Mew was not ordinarily obtainable. Others confuse it with the 100 new species introduced in Pokémon Gold and Silver. The number 151 refers to the complete Generation I Pokédex, including Mew.