See ALL 150 Pokemon in the Sinnoh Dex, then go to anyPokemon center and access the PC. Go on to "Rowans PC" and he will give you the National Dex.
You have to see all 150 Pokemon to do that u must beat the elite four.
First of all it depends of what game your playing mostly you will need to register over 60 Pokemon or more in your pokedex then show it to the professor in that game, if that's not the case you may need to see all of the Pokemon in that particular region.
You need to complete the game by beating the Elite Four and your rival and see all the Pokemon in the local dex once, then you can get the National Dex.
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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