The original first generation contained 151 Pokémon. These species debuted in the 1996 Game Boy games Pokémon Red, Green, and Blue.
The sequence runs from Bulbasaur at National Pokédex number 001 to Mew at number 151. Mew was hidden in the original games and was not normally obtained through ordinary gameplay, but it was still included in the complete first-generation count.
A common mix-up is the number 150. The first games’ standard Pokédex entries ran through Mewtwo at number 150, while Mew occupied number 151. Another common confusion comes from counting only Pokémon that could be obtained through regular in-game methods.
Later generations expanded the National Pokédex. Generation II added 100 new species, bringing the numbered total to 251. The first-generation total nevertheless remains an important milestone in Pokémon history and marketing.