IMO, Shadowrun's magic system is the most "realistic" of any I've ever seen ("realistic" meaning I could actually imagine magic working that way if it were real). Shadowrun's magic system is very different from spells per day or spell-point systems. There is no set limit on how many spells you can cast. Instead, whenever you cast a spell, you must roll to resist "drain". Drain causes stun damage, which is basically fatigue. The more powerful or complex spell you cast, the greater the potential drain. It is possible to entirely resist the drain from a spell with a good roll. It's also possible to cast spells that exceed your normal limitations. In that case, the drain causes physical injury instead of stun damage. A mage can, in fact, kill himself by trying to cast a spell too far above his limits.