Pokémon Generation II introduced 100 new Pokémon species.
Generation II began with Pokémon Gold and Silver, followed by the enhanced Pokémon Crystal. These games expanded the original collection of 151 species to a total of 251. The new Pokémon range from Chikorita, the Grass-type starter, to Celebi, a Mythical Pokémon associated with time travel.
The second generation also introduced the Johto region, which is connected to Kanto. After completing much of the Johto adventure, players can travel into Kanto and revisit many familiar locations. This made Gold and Silver feel like a continuation as well as a larger adventure.
Generation II added more than new creatures. It introduced Pokémon breeding, a day-and-night system, held items, the Dark and Steel types, and color support designed for the Game Boy Color. The figure of 100 refers to new species introduced in the generation, not the number of Pokémon obtainable in every single version.