I've run as many as 8 or 9 on a regular basis. I have a few rules and tips that work:
1) Cut down on the chatter as much as possible. This many players, they have to be interested in what the others players say or what they're doing. Anyone not interested in that can be bused out of the game, solving the too many players problem.
2) No one gets to do something on their own, unless it requires less than three minutes to run. Someone wants to head out and buy some equipment? No problem. They want to go off and adventure on their own? No, absolutely not. If nine people are going to play, they will play together. Those who cannot play together can get out.
3) No one rolls a die without being told to first. Having nine people spontaneously roll dice and announce results, it's impossible.
4) If something needs to be looked up, have a player do it.
5) Keep descriptions very short.
6) Learn to talk faster or use less words. Make the point perfectly clear, but don't spend more than 20 seconds making it.
7) When asking someone what they want to do in combat, announce who the next person is going to be asked and tell them "So and so is on deck."
8) Give people 12-20 seconds to say what they want their characters to do; if they hesitate or just can't decide, tell them their character is undecided and move onto the next person. This can be stretched out for situations where the decision is really critical and several character's lives hang in the balance. The other players won't mind waiting several minutes if this is the case.
9) Shut down any jokes that do not directly relate to the action; shut down any and ALL cultural references, reminisces the player will want to make about themselves or comments designed to be clever or snarky. Be firm. Don't expect to be able to keep a lid on this for more than 15-20 minutes if the players are unused to this many participants.
10) When the jokes and side comments start happening a lot despite your efforts, call a break. Every 20 minutes if need be. Keep breaks short. Sometimes five minutes is enough for everyone to have a laugh and be willing to focus again.
11) If this above doesn't work, ask three people to go home.