In the United States, full-time enrollment at the undergraduate collegiate level is 12 or more credit hours per semester. A graduate student must be enrolled in at least nine credit hours per semester. Requirements for doctorate degrees vary based on the university.
Each course's credit is dependent on the hours spent in that class. For example, a science lab class may be worth four credit hours, but a weightlifting class may only be assigned two credit hours.
When planning out a semester, it is important to look at the total credit hours that will be earned, not the number of classes that will be taken.