Once a long time ago (Sakura Con 2001) I ended up staying in a pretty small room with 8 people. That was a "share a room or don't get to go at all" situation, though, as we'd waited until the last minute and ended up sharing with some friends who already had a reservation.
The worst I've ever had for one night was 9 people, (at Sakura Con again, several years ago). However, that's because there were some minors (17 years old, each of them ><; ) who had been foolhardy enough to come to the con and plan to spend three full nights AWAKE with no room to stay in. It was 1am and no one else was still up, so our choice was to allow them stay in our room or let them be forced off of the hotel grounds without transportation.
Fortunately(?), they were embarrassed enough that they wedged themselves in a corner and didn't take up any real space. And we had a pretty big room, to begin with. It was only for one night.
This year at Kumori Con I wasn't in a terribly overstuffed room. The main problem with room-sharing, besides the mess, is people waking up and going to bed at different times. I wish I didn't have so many friends who insist on arranging groups of more than 6 people.