The original Pokédex in Pokémon Red and Blue listed 151 Pokémon.
The number includes Mew, the mythical Pokémon numbered 151, although Mew was not normally obtainable through standard gameplay in the original releases. The first 150 entries run from Bulbasaur to Mewtwo, while Mew completes the original National Pokédex sequence.
A common mix-up is saying that the games contained only 150 Pokémon because Mew was hidden from ordinary players. The first generation is therefore usually described as having 151 Pokémon, even though most players encountered 150 through regular game content. Later generations expanded the Pokédex by adding new species.