Generation I introduced 151 Pokémon species.
These species debuted in the original Japanese Red and Green games and formed the first National Pokédex. The sequence begins with Bulbasaur at number 001 and ends with Mew at number 151. Mewtwo is number 150, so Mew’s position is not evidence that it was an ordinary late-game catch.
The first-generation total is often remembered as 150 because Mew was distributed as a rare Mythical Pokémon rather than normally encountered during the main adventure. Nevertheless, Mew is part of the original 151-species Pokédex. MissingNo., a famous programming glitch, is not an official species in that count.
Later releases added new Pokémon generations, regional forms, and alternate forms, making the National Pokédex much larger. The original 151 remain especially important to the franchise’s identity because they were the creatures that introduced Pokémon to players worldwide.