You have to catch 151 pokemon to complete the pokedex.
catch all Pokemon
you don't need to catch them just see all the Pokemon in the hoenn pokedex
When you complete your pokedex, you'll know
aftewr you get your national pokedex you will get a place where you can catch it only of you have the bayleaf on Ruby.
first you have to catch and evolve all the Pokemon in ruby. then if you want to complete the pokedex, you have to trade for all the other Pokemonsee http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_Ruby_and_Sapphire_Versionsfor more info
to get the jirachy u must have all 200 Pokemon (complete pokedex) and you you now have it to get the jirachy u must have all 200 Pokemon (complete pokedex) and you you now have it
Yes the only catch is you can catch certain Pokemon in ruby that sapphire can't get and vice versa but the pokedex is the same.
Emerald.
you can check the pokedex, it is in fiery path.... and many caves! Just check the pokedex, it's quicker.
go on codejunkies.com to find out
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.
you can't unless you get the national pokedex and trade for one in sapphire or ruby.