Pokémon Gold and Silver introduced 100 new Pokémon.
The second-generation games expanded the original roster of 151 with creatures numbered 152 through 251 in the National Pokédex. They were released in Japan for Game Boy Color in 1999 and added the Johto region, a day-and-night cycle, breeding, held items, Pokémon eggs, and two new types: Dark and Steel.
Gold and Silver also connected Johto to Kanto, allowing players to revisit the setting from the first games after completing the main story. This made the sequel feel substantially larger than a simple regional follow-up.
The number is sometimes confused with the total number of Pokémon available at launch. The games contained 251 species in the National Pokédex, but exactly 100 of those were newly introduced in the second generation.