catch all Pokemon
http://pokemondb.net/pokedex/national
beat the hoenn pokedex and go to prof.birch
The original Pokedex is given to the player by Professor Birch. After the Pokemon Ruby trades with LeafGreen or FireRed the National Pokedex will be unlocked.
Unfortunantely, it's impossible to catch ANY Pokemon from the National PokeDex in Ruby and Sapphire. You have to trade with FireRed, LeafGreen, Emerald, Colosseum, or XD to get Pokemon from the National PokeDex. If you trade with the first three games, you'll actually get the National PokeDex in Ruby and Sapphire.
I believe there's 202 in the Hoenn pokedex specifically, and 386 in the national.
you have to get 150 different types of Pokemon
trade it from ruby sapphire or emerald after getting the national pokedex.
After becoming champion of Hoenn by defeating the pokemon league
aftewr you get your national pokedex you will get a place where you can catch it only of you have the bayleaf on Ruby.
I believe there's 202 in the Hoenn pokedex specifically, and 386 in the national.
you can't unless you get the national pokedex and trade for one in sapphire or ruby.
You have to catch 151 pokemon to complete the pokedex.