There are 493 Pokémon in Brilliant Diamond's Pokédex. Only 484 are needed to get the certificate from the Game Freak employee on Valor Lakefront; the ones that can be missed are Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, and Arceus, which are all only obtainable with the use of events. This page focuses on getting the other 484 Pokémon.
In order to complete the Pokédex, you will need access to at mininum a copy of Pokémon Shining Pearl.
If the player is able to deposit the 150 Pokémon of the Sinnoh Pokédex (excluding Manaphy) into Pokémon HOME, with each Pokémon originating from a copy of Brilliant Diamond or Shining Pearl, the player can get a shiny Manaphy as a reward.
The following version-exclusive Pokémon have to be traded from Shining Pearl:
17 Pokémon evolve by trading: Golem, Machamp, Alakazam, Gengar, Politoed, Slowking, Steelix, Scizor, Kingdra, Porygon2, Huntail, Gorebyss, Rhyperior, Electivire, Magmortar, Porygon-Z, and Dusknoir. The easiest way to get data for them is to trade them to another game and immediately trade the newly-evolved Pokémon back.
You will need two King's Rocks, two Metal Coats, one Dragon Scale, one Up-Grade, one DeepSeaTooth, one DeepSeaScale, one Protector, one Electirizer, one Magmarizer, one Dubious Disc, and one Reaper Cloth to evolve Politoed, Slowking, Steelix, Scizor, Kingdra, Porygon2, Huntail, Gorebyss, Rhyperior, Electivire, Magmortar, Porygon-Z, and Dusknoir.