I think it also depends on the timeframe for the courses. If you take traditional semesters, where classes last approx. 16 weeks, I think I could easily handle 4-5 courses at one time. My school is non-traditional with semesters at th 16-week schedule, but my lit courses are all 16-week courses whereas my English and history courses are 8-week courses, which include as much reading as a 16-week course. So, I take at least 4 courses in the 16-week semester, but only take 2-3 at a time, staggering their start dates.