Biggest collection of interviewers I had was eight, set up like a panel discussion, for a govt communications post. The group included the supervisor, the supervisor's director and a smattering of colleagues I would be working with. Felt like I was on Meet The Press, but more confrontational.
When dealing with a government job, take the number of people required to get it done properly and multiply by at least four.
I also had four interviewers at once in a conference call phone interview for a university job.
Interesting common thread: Both jobs were never filled because of budget cuts. And I suspect both groups suspected as much. Seemed more like an exercise.