You get the National PokéDex after seeing the 150 Pokémon in the Sinnoh Dex. After that go to Professor Rowan in his lab in Sandgem Town and talk to him and then Professor Oak will come in and present you with a National Dex as a gift.
If you have been looking around on the internet (more directly www.serebii.net) they will tell you that you need 151 (the +1 being Manaphy), Manaphy is only required in Platinum. -TyCundy
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There are 210 in the Sinnoh Dex, 493 in the National Dex {including EVENT Pokemon}
The last Pokemon on the national dex is Arceus.
Piplup's National Dex number is 393.
In Pokemon Platinum to get the National Dex you will need to have seen all 210 Pokemon in the Sinnoh Dex then talk to Professor Rowan and then after a while Professor Oak will give you the National Dex!
from proffeser rowen after you get the sinnoh dex filled