The original Pokédex of Pokémon Red and Green listed 151 Pokémon species.
Those 151 species formed the first generation, commonly associated with the Kanto region. The list ran from Bulbasaur at number 001 to Mew at number 151, although Mew was not normally obtainable through standard gameplay in the original releases.
The initial set included familiar Pokémon such as Pikachu, Charizard, and Meowth. Version-exclusive species meant that players could not complete the Pokédex using only one copy of the game, making trading an important part of the original design.
Later generations expanded the National Pokédex far beyond the first 151. The number is therefore specific to the original generation, not to every Pokémon available in later games. Some early promotional material and game events treated Mew differently from the normally obtainable Pokémon, but it remains number 151 in the original Pokédex numbering.