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.
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.
None at the moment because I work full time for the Air Force doing Fitness Assessments, take classes at two Universities, and have a brand new baby daughter.
Last year I averaged 6 in my one hour boot camp slot, had 2 slots always full for one on one personal training, and had between 5 and 10 slots per month filled with movment assessments and program design for referral clients (this was a two session deal, one for assessment and one for going over the program.