Pokémon Red, Blue, and Yellow introduced 151 Pokémon species in the series’ first generation.
These original species are numbered from Bulbasaur at National Pokédex No. 001 through Mew at No. 151. The games established the idea of completing a Pokédex by finding, catching, evolving, and trading Pokémon across two connected Game Boy cartridges.
Although players could encounter many Pokémon during normal play, completing the original Pokédex required trading between versions. Some species were exclusive to one version, while others evolved only after being traded. Mew was also part of the original 151, despite being hidden from ordinary gameplay at launch.
A common mix-up is saying that the first games contained only 150 Pokémon. That number usually excludes Mew, which was treated as a secret Mythical Pokémon. The number 151 therefore refers to the complete original species list, from Bulbasaur to Mew.