Only limit in place is the level of the skill, which peaks at level 10. You'll need enough Spirit+Coins to learn all the skills up to level 10. And then, you'll need to learn level 11 (Demon/Sage skill) which can be bought from the boutique/relevant catshops around the server, and these are learnt in heaven/hell. Morai skills can be bought/gained from questing in Morai, and can be learnt anywhere. And AEU/Ares skills can be bought/gained from going to Ares every Saturday, or by doing AEU (Which no-one does anymore) and can be learnt anywhere.