After defeating Team Plasma and seeing the credits you will be taken back to your home. Once all the talking inside the house is done you go outside and Professor Juniper's father should come and give you the pokedex
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The last Pokemon on the national dex is Arceus.
There are 210 in the Sinnoh Dex, 493 in the National Dex {including EVENT Pokemon}
you defeat team plasma and get the national dex,then battle alder
What you have to do is catch all the Pokemon from all the other regions(complete national dex.
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!