The original Pokédex contained 151 Pokémon. The first-generation games introduced the species numbered 1 through 151, ending with the Mythical Pokémon Mew.
The first 150 entries could generally be encountered or obtained through normal gameplay, depending on the version and trading requirements. Mew was listed as number 151 but was not normally obtainable through standard play in the original releases.
Mewtwo is number 150, which causes one of the most common counting mistakes. Some players also remember 150 because Mew was a secret addition and was not widely known when the games launched.
Later generations expanded the Pokédex with new regional groups and species. The number 151 therefore specifically describes the original generation, not the total Pokémon species across the modern franchise.