The first Pokémon generation included 151 Pokémon.
Generation I introduced the species available in the original Japanese Red and Green games and the later international Red and Blue releases. The final numbered entry was Mew, although it was hidden and not normally obtainable through standard gameplay when the games first launched.
The number 151 is often confused with 150 because Mewtwo is the last Pokémon obtainable through ordinary in-game progression and is numbered 150 in the original Pokédex. Mew is number 151 and was distributed through special promotions rather than regular encounters.
The first-generation Pokémon are associated with the Kanto region. Later generations expanded the Pokédex with new regional groups, eventually making the original 151 only the beginning of the franchise’s creature roster. Anniversary references to the “original 151” specifically mean the Pokémon introduced in Generation I, not every Pokémon available in the broader series.