There will be two ways of magic:

Casting by using a wand, where the wand has a special amount of uses (e.g. fireballs). This magic does not require any mana and the wands can be reloaded by magic merchants for gold.

The other way of magic is the use of magic scrolls. You choose them like a weapon and can cast the spell as long as your mana lasts. You can recharge your mana with mana potions, one kind refills half the mana, the other full mana.