Mani (Mah-nai) are the creation of Grathis, as detailed in the Beasts racial segment. They are microscopic and EVERYWHERE. You breath in, you breath in some Mani. They live under the water, in the air, in the ground, in fire, in acid, everywhere. And they have absolute power of illusion.
A Tal'Vornian Spell is just that, an illusion. However it is so perfectly crafted that the human/Elder body actually reacts to the illusion as if it where real. And no you can't avoid spells by saying 'Ha Ha, I don't believe in your illusion' The upshot is that the Mani reproduce by creating an illusion which causes them to split. So when someone casts a spell, the image they have in their mind, the Mani copies and brings into being.
So the amount of spells you can cast are limited by only one thing. The amount of Mani in the area. And since it takes dozen's of Mani to create one spell and the Mani cannot divide twice in rapid succession, you CAN eventually, run out of Mani.
For clarities sake it is worth mentioning that no-one, Caster or otherwise, is aware of the existence of Mani or the exact method with Magic functions.