Generation I of the Pokémon franchise introduced 151 Pokémon species.
The original group appeared in the 1996 Game Boy games Pocket Monsters Red, Green, and Blue. Their National Pokédex numbers run from Bulbasaur at 0001 through Mew at 0151. This group is often called the original 151.
The number includes the legendary and mythical Pokémon introduced in that generation, including Mewtwo and Mew. Mew was number 151 even though it was not normally obtainable through standard gameplay when the original games launched, which helped make it especially mysterious to players.
A frequent mix-up is saying 150 because Mew was hidden and difficult to obtain. Another is counting only the Pokémon visible in one version, since Red, Green, and Blue had version-exclusive species. Taken across the Generation I games, the official total was 151.