A Trainer can earn eight Gym Badges in Pokémon Red and Blue.
The badges come from defeating the eight Gym Leaders located across the Kanto region. Each Gym specializes in particular Pokémon types or battle strategies, and the badges serve as proof that the player has overcome those challenges.
After collecting all eight badges, the player can enter the Pokémon League at Indigo Plateau. The League challenge consists of the Elite Four followed by a final battle against the player’s rival, who serves as the Champion in the original games’ story.
The total is sometimes confused with the number of Elite Four members, which is four, or with the number of Pokémon species in the first-generation Pokédex, which is 151. Later Pokémon games generally retained the eight-badge structure while changing the regions and Gym Leaders.