This is sort of unanswerable, as there are SOOOOO many different factors that numbers of plans are sometimes nearly (but not totally) meaningless. Size of plans, complexity, combined plan testing, support staff assistance (if any) "user friendly" systems and procedures, investment platforms, quality of client HR personnel, TPA services performed, etc., etc. - some plans are giant time-sucks, and some are relatively smooth. You could go on and on.
Having said all that, I would say that 120 plans would typically be considered on the high side, but on the other hand, if he's been able to handle it without working an excessive number of hours per year, then it probably isn't "too many."
Good luck with whatever you decide.