Pokémon Gold and Silver added 100 new Pokémon species to the second generation.
The second generation began with the Japanese release of Gold and Silver in 1999. The games introduced the Johto region, new characters, new story events, and the ability to revisit the earlier Kanto region after completing the Johto adventure.
The newly added species ran from Chikorita through Celebi in National Pokédex order. With the original 151 species included, the total Pokédex count reached 251. Gold and Silver also introduced Pokémon breeding, a day-and-night system, held items, and the Dark and Steel types.
Pokémon Crystal was released later as an enhanced version of the same generation. The number is sometimes confused with the number of species in later generations, but 100 is specifically the second generation’s addition, not the total number of Pokémon available in Gold and Silver.