Generation I of the Pokémon games introduced 151 species.
These Pokémon first appeared in Pokémon Red, Green, Blue, and Yellow, whose setting is the Kanto region. The National Pokédex numbers them from Bulbasaur at #001 through Mew at #151.
A common source of confusion is the number 150. Mewtwo is #150, but the mythical Pokémon Mew is #151. Mew was not normally obtainable through standard gameplay in the original games, yet it is still part of Generation I’s official Pokédex.
Later generations expanded the total dramatically. Generation II added 100 species, bringing the running total to 251. The first generation remains especially recognizable because it supplied the original starter Pokémon, the original Eeveelutions, and many of the franchise’s most famous species.