<p>You don’t need any more classes to “fill your unit load” if you already are at 12 units. You only need 12 units to be full time. 13 is the average you have to meet over the entire year. In other words, you need 39 units for the year; if you take 12 units in the Fall, you have 27 left for the Winter and Spring quarters. Divide them up however you want, but 12 is the magical full time number for any particular quarter, not 13.</p>