The original Generation I games introduced 151 Pokémon.
Pokémon Red, Green, Blue, and Yellow are associated with Generation I, whose setting is the Kanto region. The Pokédex numbering runs from Bulbasaur at number 001 to Mew at number 151. The first 150 were generally available through normal game content, while Mew was distributed through special events in the original era.
A common mix-up is saying that the first games had only 150 Pokémon. That figure excludes Mew, whose existence was hidden from most players at launch. Another source of confusion is the Japanese release: Red and Green came first, while Blue was later used as the basis for international versions.
Generation II added 100 more Pokémon, bringing the total to 251. The original 151 remain especially important to the 30th anniversary because they define the franchise's first generation and its earliest global identity.