The first Pokémon generation introduced 151 species.
These Pokémon are numbered from Bulbasaur at National Pokédex No. 001 to Mew at No. 151. They appeared in the original Red, Green, Blue, and Yellow games, although some could only be obtained by trading between versions.
The number 151 includes Mew, even though Mew was not normally obtainable through standard gameplay in the original releases. This is why people sometimes remember the first games as having 150 Pokémon while the complete first-generation count is 151.
The first generation established many enduring characters, including Pikachu, Charizard, Jigglypuff, and the three starter Pokémon Bulbasaur, Charmander, and Squirtle.