The capacitance of a capacitor is fixed once it is constructed, as the only way to change the capacitance is to change its "build" parameters: the area A and/or the separation distance d.
So just how much charge could a capacitor store on its plates? When does a capacitor "know" when to stop charging?
(The units for charge (in coulombs, C) and potential (in volts, V; or joules/coulomb, J/C) again work out to coulombs2 per joule, or farads for the capacitance.)
The key is to realize that the capacitance of a capacitor, once built, is fixed. However, one can change the amount of potential applied to the capacitor (by connecting different batteries, etc.). With a given value of capacitance, then applying a high potential to the capacitor will allow it to store more charge, and applying a low potential to the capacitor will have it store less charge.
Thus capacitance can be said to be a measure of "charge-storing efficiency" with respect to a given potential, in that a large capacitance capacitor will store more charge than one with a small capacitance, if they are connected to the same potential source (such as a battery).
Thus the first electron moved costs nothing (or nearly nothing), while the last electron requires a much higher q·∆V cost to move. The average cost of moving each electron, then, can be said to be (1/2)·q·∆V, such that the total cost of moving all electrons is (1/2)·Q·∆V, where the total charge of all N electrons moved is Q = N·q. (This argument is deliberately trying to avoid calculus to integrate the gradually increasing cost of each electron over all electrons moved, but the result is the same.)
Endless hours of amusement await you when solving capacitor energy problems, so use caution when you use these equations, and more importantly, use only the equation you really need.