Generation IV introduced 107 new Pokémon species in the Nintendo DS games Pokémon Diamond and Pearl.
Those games were released in Japan in 2006 and took place in the Sinnoh region. The new species were assigned National Pokédex numbers 387 through 493, beginning with Turtwig and ending with Arceus.
The count refers to new species introduced by Generation IV, not the total number of Pokémon available in Diamond and Pearl. The games also included many species from earlier generations, and later releases added further Pokémon and forms associated with the generation.
Some Generation IV Pokémon appeared in other media before Diamond and Pearl launched. Bonsly, Mime Jr., and Munchlax, for example, had already appeared in the anime. That early exposure can make the generation’s debut history seem less straightforward, but the official generation count remains 107.