[1][2][3] A few systems that use MP do not have a maximum number that may be stored, but instead make it more difficult to recover or gain new magic points.
Examples of MP-limited systems include Rolemaster, High Adventure Role Playing, GURPS, and Tunnels & Trolls.
Such systems are often limited by an increase in the difficulty of the skill roll based upon the number of spells in a certain time period that have already been cast.
For example, Dungeons & Dragons simplified Jack Vance's formula "to a number of spell slots scaling with the player character's level".
[5]: 28 As an example, Mage: The Ascension uses a skill-limited system that may be augmented by spending Quintessence to lower the difficulty of a magical skill roll.
Ars Magica uses a skill-based system, but a mage can only cast so many spells before becoming too fatigued to continue.