You need to get 100 pokemon then go and talk to professor Birch and he will upgrade your pokedex.
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You can get Charmander in Pokemon Sapphire by obtaining it in Pokemon FireRed or LeafGreen and trading it to Sapphire. To have Charmander registered in the Sapphire PokeDex, you must defeat the Elite Four. You may have the Charmander before that, but it won't register in the PokeDex.
Route 208, Route 210 (After obtaining the National Pokedex, insert Sapphire cartridge into your DS).
There are currently 493 in the National Pokedex.
In Pokemon Diamond and Pearl you need the National Pokedex to get Magnemite. In Pokemon Platinum you don't need the National Pokedex to get Magnemite.
You finish the Pokemon league and get the national pokedex You finish the Pokemon league and get the national pokedex