It's also necessary to differentiate between semesters and terms (my schools divide terms by the seasons, so there are four a year, but really only three per school year) and between 3 and 4 credit classes: I've been taking 3 credit classes, so that means four classes total at full time. If I take 4 credit classes, I may be able to squeeze 16 credits, but it will very much depend on the class and difficulty.
This last term I was taking sociology, African American literature, art history, and state and local government. It was a full plate.
AFAIK, at the University (which I have not yet attended but will this fall), if you take 16 credits you essentially get a free class. That may be worth it.