The original Pokédex in Pokémon Red and Blue contained 151 Pokémon species.
That total covers the first generation, beginning with Bulbasaur at National Pokédex number 001 and ending with Mew at number 151. The games' central collection goal was to obtain every species, although completing the Pokédex required trading because some Pokémon were exclusive to one version.
The number 151 is also why Mew occupies the final place in the original Pokédex. Mew was not normally obtainable through standard gameplay in the first releases; Nintendo later distributed it through promotional events. This led to confusion about whether the original games officially contained 150 or 151 Pokémon.
Pokémon Gold and Silver expanded the Pokédex to 251 species by adding 100 more. Thus, 151 refers specifically to the first generation, not the total number of Pokémon across the modern franchise.