Yes. Master Magic Materia is ONE materia with EVERY spell. So the slots in your weapons/armor don't matter. You should have plenty of room for one materia since you wouldn't have to equip ANY other magic materia after that. I know what you meant. Which is why I said "sets" of spells. i.e. the fire spells, the lightning spells, the gravity spells, etc. There's 20+ sets in the guide. And the dude has one materia that has all of them. Its not cheating. You can get that materia by defeating the Weapons (I forget which one) or make your own by mastering every spell materia in the game. (Note: You can do the same thing with other materia for a master summon materia and master command materia)