Right now i average between 6-10, but i also do a host of other things in my gym (TRX group classes 5x week, instruct college PE courses, programs, seminars). I find any more than 10 clients at one time will not allow me enough time to focus on good programming, client relations, and detract from my other obligations.
I seem to have 5 to 8 very regular, long term clients all of the time. And then 5 to 10 clients that want to address a specific goal and learn to exercise on their own. I also teach students preparing to take a cert exam and enrollment is up and down throughout the year. And I teach CECs about three times a year to small groups of about 8 for 2-3 days over a weekend.