The original 1996 Pokédex contained 151 Pokémon.
That first group ran from Bulbasaur, number 001, to Mew, number 151. The creatures appeared in Pokémon Red and Green in Japan, with the same original generation later reaching international players through Red and Blue.
Mew was especially unusual because it was not normally obtainable through standard gameplay. Its hidden status helped create lasting rumors about how to find it, while official distributions eventually made the mythical Pokémon available to players.
The number 151 applies to Generation I, not to the entire modern Pokédex. Later generations added new regional groups, so the total grew well beyond the original Kanto roster. Mewtwo is number 150, followed by Mew at 151.