The original National Pokédex contained 151 Pokémon.
These species are now commonly called Generation I Pokémon. They first appeared in Pokémon Red and Green in Japan, with numbers running from Bulbasaur at No. 001 to Mew at No. 151. The original set includes the three starter families, iconic species such as Pikachu and Eevee, and legendary Pokémon such as Articuno, Zapdos, Moltres, and Mewtwo.
Mew’s inclusion is an important part of the number. It was not normally obtainable through ordinary gameplay in the original releases, but it was assigned Pokédex number 151 and was distributed through special events. Mewtwo is No. 150, so the two are often reversed by mistake.
The term “National Pokédex” later came to describe a complete species list that includes Pokémon from every region. In the first generation, however, the original Pokédex and the National Pokédex contained the same 151 entries. Later games increased the total, beginning with 251 after Gold and Silver.