Pokémon Diamond and Pearl added 107 new Pokémon to the National Pokédex.
Released for the Nintendo DS in Japan in 2006, Diamond and Pearl introduced the fourth generation of Pokémon. Their new species included the starters Turtwig, Chimchar, and Piplup; the legendary mascots Dialga and Palkia; and many evolutions of earlier Pokémon, such as Rhyperior, Magmortar, and Togekiss.
The additions brought the National Pokédex total from 386 species in Generation III to 493 after Generation IV. That total includes the 107 species introduced in Diamond and Pearl plus Pokémon from earlier generations.
The number is sometimes confused with the number of Pokémon obtainable in a particular regional Pokédex. The Sinnoh Pokédex was revised in Pokémon Platinum, and its entries did not represent every species introduced by the generation. The question asks about new species added to the National Pokédex, making 107 the precise answer.