lou,
There is no magic ratio, but for many the range is 15-20 students, However, that can vary greatly depending on the type of course, the level of the course content, the type of assignments and design of the course, etc. For example, a writing course should probably have fewer students since the amount of time to provide quality feedback to the students is greatly increased. The research and discussion focus of many graduate courses might also suggest smaller numbers for those classes.
Herbert Brown III
