One would think magic is simple, but it is not. It's not just waving your hands in a funny fashion with a few silly words to make a fireball. No, this is mostly mental control and formulas. Occasionally there's a verbal command involved, but very few somatic or material components, unlike D&D.
But before we describe spell casting, we should discuss mana, the source of magic. Mana is everywhere. Some would describe it as the source of life, but this is not true. Otherwise everyone would be dead after the Mana Wars when magic all but vanished from the world (thank you cataclysmic events for that one). However, mana is a source of energy that is prominet throughout the world, but some areas have more than others, and while a few rare locations are void of it. The former surplus is the result of Leylines, walls of pure mana the criss-cross throughout the world. They are visible during the night, appearing not unlike the Aurora Borealis. The later is refered to a Dead Zone: an area with no magic and where magic cannot be used. Nobody knows why.
The next important thing to go over is how mana interacts with people. All humanoids (humans, elves, dwarves, orcs, etc) have magic circuits - a special nervous system just for using mana. Now, the average human only has a few (we're talking low single digits here), while elves have more (low 10's). Mages have a great deal of magic circuits (anywhere from 20-50), while certain creatures have even more (some of the ancient golems have been discovered with over a hundred circuits). In game system terms, a character with a high Energy Point pool will have more magic circuits than one with a low EP. Regardless, to use magic or even magic items, one must have mana. By drawing from those magic circuits while applying certain magical programs, one can cast spells.
The tricky part about spell casting is that they're actually just magical programs. You have to 'teach' your magic circuits to use this spell before you can actually use it. Which is harder than you'd hope. This can take months, if not years, depending on the spell. The more complex and powerful, the more time it takes for your circuits to learn the spell. Afterwards, it's a matter of recalling the correct mental commands and directing mana through the right circuits, and the spell is cast.
This is where magic items, especially MysTek Artifacts, come into play. These items have spells already programed into them, thus you just have to supply the mana to cast them. Even constant effects may only require the essence of mana to be powered. This is also how Dragonite is used - it possesses magic circuits that can only produce heat.
In all reality, most mages only know a dozen spells or so. Only Archmages will know a lot. It really comes down to the use of the spells, not the number, that's important. Spells don't always have a singular use. For example, a telekinesis type spell can be used to move something, or converted into a barrier to use against attacks.