every dojo has a diffrent number of students depending on the location, style, advertising, ect. trying to get an average for something so broad would be insanly hard as well as pointless

Enough students to defray the costs of maintaining the dojo without compromising the quality of instruction or the integrity of the art.

There are far too many variables to be able to average this. Some styles are designed for training large armies and as such may have over a hundred in the all day, every day class. Some styles are more about training a few good defenders for the people of the area and keep the size limited to ten or less, again in an all day every day class. Some styles are just about having some fun while learning some things about simple self defense and as such the classes tend to be around twenty students ... but the dojo may have many classes ... like perhaps a two hour morning class, a two hour afternoon class and a two hour evening class for three days per week ... then a second set of classes on the other two weekdays ... then perhaps a weekend practice session ... so a school like this may still have over a hundred students in its six classes ... Some styles prefer the instructor to teach only private and semi private lessons to one or two students at a time ... I personally will only train up to ten per class and prefer only four students at a time. I teach three hour classes three days per week, plus open practice sessions for five hours on Saturdays & Sundays ... for only 15 hours per week ... but then I also have a second company for more money.