I'd guess three or four using a camera at one time would be the most before people start to feel left out.
If you've got three or four people on a camera, that's enough people to remember everything that needs to be remembered to get an accurate exposure with a light meter. If they're using exposure record sheets, which they should while they're learning, then the sheet can also serve as a checklist for: base exposure, filter factor, bellows factor, and reciprocity, setting aperture and shutter speed, lighting ratio for studio, scene brightness range for outdoors, and Zone placement and development plan, if you're using Zone System controls.