Generation I introduced 151 Pokémon species.
These original species appeared in the 1996 Game Boy games Pokémon Red, Green, and Blue, with Pokémon Yellow following later in Japan. The sequence begins with Bulbasaur at National Pokédex number 0001 and ends with Mew at number 0151.
The figure 151 is closely tied to the franchise's early identity. The first games encouraged players to collect Pokémon across two versions, because some species were exclusive to one version and had to be obtained through trading. Mew was especially famous because it was not normally obtainable through ordinary gameplay in the original releases.
Generation I is sometimes described as the “original 150” because Mew was initially distributed separately and kept secret. The completed National Pokédex count, however, is 151, making 151 the canonical answer.