Heh... if all the players show (happens every now and again) I think there are ten players in my game.
We run two campaigns concurrently, a halfling-themed game and an epic-level out of hand crazy game (story hours in sig). The same players are in both, and we sometimes play several sessions of one before switching back to the other. Presently, for instance, we're playing a lot of epic games (though the next session we play the halflings).
Setting: My apartment, with gaming maps on all the walls. (Yeah, I'm a geek.) The prize one is the one that one of my most artistically-inclined players ever watercolored for me. We're gonna do more of those soon.

There is a couch that sits before a low coffee table, upon which rests my
box of dice; on the other side of that is a large round table covered in miniatures, behind which are my
shelves of gaming. Two players usually sit upon the couch. Most of the rest take plastic lawn chairs, though there is also an office chair should we have that many people at once. My chair faces the rest of the room, and is flanked by the battlemat's table (usually dotted with recently-used miniatures).
Ambience: Fun times! We drink a ton of beer, smoke a lot of Old Tobey and generally have a blast. Our last three sessions have been twelve hours long, and despite the party atmosphere
lots gets done in a typical game.
Regularity: We play frequently but irregularly. It really depends mostly on my schedule, as dm and host, but I try to accomodate my players as much as possible. Being irregular, and having so many players, almost everyone sometimes misses a game, which leads us to...
Quorum: I saw someone else above mentioned that two gamers missing = let's play board games instead. To my mind, one of the great advantages of a large group is that no one person being missing can spoil the game. We consider dm + 3 people to be a quorum, and sometimes less (especially if whoever's early has some solo stuff to do or whatnot). Interestingly, in a long session sometimes there are eight people who play, and four of them never see each other.
Style: The high-level game is truly epic in style, with assassination attempts funded by Asmodeus on the pcs, major world war, high magical disasters (the ol' 3.0 multi-empowered simulacrum trick...), etc. The halfling campaign is more comedic- from the halfling with a goblin wife to the infamous episode with the crossroads, it's a laugh a minute.
System: We play 3.5 with a few house rules. We use most of the WotC stuff, and I have a lot of homebrewed goodies. I usually award xp at the start of the game for the session before, because we play til we're dropping most of the time. (Our enthusiasm runneth high, can you tell?) Once in a while I give it out at the end, though, and during the Phat Weekend of Gaming (we played 24 hours between Saturday and Sunday) I gave xp twice each day. (
That was a lot of gaming!)