As supermtt implies, credit hours is more important than number of classes. For instance, a 1.5 credit class is barely noticeable on my schedule while a four credit course takes up a lot of time. Most courses are 3, with some science adn math being 4.
Full time is between 12 and I think 20.5 credits hours. However, since an average class is 80 minutes and meets twice a week, the actual time in class is a bit less. So far, I have taken anywhere from 13.5 to 17 credits.