After beating the Pokemon league, professor oak will look at your pokedex. If you have caught at least 60 Pokemon, he will take you to his lab. If not, then go catch at least 60 Pokemon and go the lab yourself. He will tell you about how Pokemon from johto and hoen are appearing. Your rival will walk in and say he should catch them. The professor will then upgrade both pokedexes to the national level.
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first beat the elite four + champion, then have at least 60 different Pokemon then see prof oak
you can get the national dex from prof. birch after deafting the the elite four. then you can trade larvitar from leafgreen or firered. you can find larvitar in the seventh island in leafgreen/firered
Go to Cerulean Cave. To get there, you must get the ruby and the sapphire and get the national dex.
The last Pokemon on the national dex is Arceus.
Electabuzz
There are 210 in the Sinnoh Dex, 493 in the National Dex {including EVENT Pokemon}