The original Generation I Pokémon games contained 151 species.
Generation I consists of the original Japanese Red and Green games, their enhanced Blue version, and Pokémon Yellow. Together, these games introduced the Kanto region and the first complete Pokémon roster, beginning with Bulbasaur and ending with Mew in National Pokédex order.
The number is often described as 150 because Mew was hidden and was not normally obtainable through ordinary gameplay in the original releases. Mewtwo is number 150, while Mew is number 151. Distribution events and special methods later made Mew available to players, but its unusual status created lasting confusion.
The original version differences also meant that no single cartridge offered every species through normal play. Players had to trade between versions to complete the Pokédex. That design made communication and exchange central to Pokémon from the very beginning, rather than being features added only in later generations.