The Lost Ring 

There were hundreds of different spells, but their number was not infinite, especially the categories into which certain spells were categorized.

Young mages usually strived to learn the most powerful spells, but very soon, they realized their mistakes that only beginners could make and allocated their Tantum to different spells to cover all their needs. 

You couldn't put everything on firepower, you needed defense, speed, and something in between with mediocre damage but equally low mana consumption. Spell allocation was to some extent an allocation of one's resources where a balance had to be found.

However, balance did not mean that all spells had to be ordinary and unremarkable so as not to overload themselves. It was possible to take some extremely simple spell to have something really powerful in your arsenal without overloading yourself.

That was what most experienced mages did because they knew what they needed in battles to win and survive.