The original Pokémon Red and Green introduced 151 Pokémon species.
These games established the first-generation Pokédex, beginning with Bulbasaur and ending with Mew in National Pokédex order. Players encountered most species in the games, while some were version-exclusive or obtainable only through trading, encouraging communication between players with different versions.
The figure 151 includes Mew, even though Mew was not normally available through ordinary gameplay at launch. Mew’s hidden presence became a famous part of the games’ history and helped fuel rumors and player experimentation. The original regional Pokédex in the games could also differ from the later National Pokédex presentation.
The total is not the same as the number of Pokémon in the original Base Set trading cards, which contained 102 cards. Nor is it the current franchise total, which has grown across later generations. For the 1996 games, however, 151 is the canonical first-generation species count.