Typical party size
My largest parties have included more than 20 role-players. I briefly ran AD&D1 for my entire dorm floor in my freshman year of college, and I ran a longer-term AD&D2 campaign with my students when I was teaching. Most of my groups, however, have under a dozen members.
When my later groups reached 15 people or more, they typically spun off into other groups based on geography and the preferences of other Game Masters. For example, I live near Boston, and for D&D3.0 in the early 2000s, gamers commuted from an hour's drive away -- as far as Worcester, Rhode Island, and New Hampshire. Several eventually found or started games closer to home.
I'm currently running three smaller groups of six people each on alternating weeks rather than one large adventuring party, since there's only so much room in my basement. Other G.M.s run parallel groups in different genres and rules systems. When my weekly telecom game has dropped below four, we've had trouble getting quorum, so I think five or six is my ideal number.