Tuition covers you to take between 12-18 credits per semester. Most students take 15 per semester. Generally, courses are worth between 1-4 credits; most are worth 3. A credit hour is based on how much time you spend in class per week (except for online classes; those are a bit different). For now, I'm taking 12 credits of in class work and 3 online (5 courses total), but am spending about 23 hours per week on campus because of the way classes are scheduled. Next semester, I will still technically be in class for 12 hours (plus 3 online credits) per week, but will likely only be on campus three days a week instead of 4 and be there 17 1/2-18 hours per week (barring work, since I work on campus) since the classes are closer together. In MD, you need 120 credit hours overall for a degree. BUT...you also need to complete all the requirement for your degree audit, in other words, the courses that are on your degree audit. I basically have left on my degree audit (after this semester) one more management course, business ethics, a lab science, one more semester of chorus, one international business course, and four business electives.
So in answer to your question, basically, by course, but it's also the amount of time spent in class each week. So if you took three 4-credit courses and one 3 credit course, it's technically only four courses but 15 credit hours. My courses for next semester are one 4 credit course, one 2 credit course (chorus), two in class 3 credit courses (one once a week and the other twice a week) and one online three credit course. But it's not based on how many days a week you're in class or how much time you spend on campus.
College is complicated.