The amount of power available to the casting player is equal to the total rolled for the Winds of Magic.

Take a number of dice equal to the Winds of Magic roll and make up a 'pool' of power dice to represent this reservoir of power. When a Wizard casts a spell, they take a number of dice from the power pool in order to make the attempt. Accordingly, each time a spell is cast, the power pool will shrink a little, thus limiting the number of spells the casting player can attempt each Magic phase.