The first generation of Pokémon introduced 151 species in the original Game Boy games.
These species are numbered from Bulbasaur at National Pokédex number 001 to Mew at number 151. The first generation appeared in the Japanese Red, Green, and Blue games and in the international Red, Blue, and Yellow releases.
The number 151 describes the original species roster, not every form, transformation, or later regional variant. Several original Pokémon eventually received new evolutions, alternate forms, or special battle forms in later generations.
A common mix-up concerns the number of Pokémon visible in a particular game. Version-exclusive species and the elusive Mew meant that completing the original Pokédex required trading or special distribution. The next generation added 100 new species, bringing the total to 251.