After defeating the Elite Four and battling all the Gym Leaders in Pokémon Shining Diamond and Shining Pearl, the goal for you will likely be to encounter all the Pokémon in the Sinnoh PokéDex. These are Pokémon that you can only encounter during the first part of the game. However, once you've found them all and filled out the Sinnoh PokéDex, you'll unlock the National PokéDex, which will give you access to all Pokémon that have appeared in previous games, Generation I through Generation III. How many Pokémon are there in the Sinnoh PokéDex in Pokémon Brilliant Diamond and Brilliant Pearl?
You can check your PokéDex at any time before completing the game to see how many Pokémon you need to find to unlock the national PokéDex. You will need to encounter 150 Pokémon in the Sinnoh region. For some, there are 151, with Manaphy being the latest.
The hardest you may encounter will be the Legendary Pokémon, the Legendary Guardians of the Lake, Chatot, Unown, and a handful of others that you may have missed because you may have traveled a different path and avoided specific Pokémon Trainer battles while exploring the game. You may also need to explore the Grand Underground a little more to find specific ones. You can find Togepi, Munchlax and Combee over there.
If you want to unlock the National PokéDex, you don't have to catch all of these Pokémon. You just need to meet them. Fighting them once in the wild or if a trainer uses one against you in a battle counts as a PokéDex entry. So, if you're missing a handful of Pokémon, we encourage you to retrace your steps to battle random trainers you may have overlooked or explore the ruins of Solaceon to meet an Unown.