You mean Masterballs? There's only one you know.
One from Giovonni, and unlimited from the lottery and trade.
Weaken it to red HP, paralyze it or make it sleep then use many ultra balls.
there are 151 between leafgreen and fire red
150
There are 2
Only 1.
Weaken it to red hp, Paralyze it or make it sleep with thunder wave or sleep powder, use many ultra balls eventually you will get it.
In total including event only Pokemon there are 10.
Only one
Of the many Pokemon that you can't get in leafgreen are: Ekans, Poliwag, Scyther, Delibird, Oddish, Murkrow, Growlithe and many more. Edit: Poliwag can be found using a super rod in the pond at route 6.
7, i believe.
There are 7 non-event islands.