My group is always between 8-13 weekly. We play every wednesday. Usually 10-12 come though. It is a little hard because only about 8 come every week. The others are a little interchangable. So we have to make up stories why they aren't there. It is really fun, but takes a lot of improv because we found the best thing to do is split in groups when things happen. Age of Rebellion made this easier because I made squad groups and have leaders of the groups. So I have a Piloting squad of 4 PCs with one of them being the Squadron Commander, a Field team (Combat), and a specializations team (Computers, engineers). It is cool because they will request players from other squads to come with them. Maybe they are going to infiltrate somewhere and the field team asks for a computers guy and a doctor to accompany them in this mission. Meanwhile the engineer will go with the Flight team. We enjoy it a lot, but you have to be able to control everything and make it interesting enough where the others want to listen even when they are not playing.

9 people in a One Ring campaign. It was a challenge but we were all good friends who were roleplaying together for a long time so it went well. I wouldn't redo it with strangers.

Cave Trolls. They were HUGE! And they didn't like it when the dice rolls didn't go their way.

Whew.... I'm a LGS owner and at my store we host a RPG day actually every weekend on Saturday. Regardless if your group is running, there are always groups being played... Or so the case has been except two other times. I've GMed for years so the play was fine. My problem was it was my 10th time actually running Edge of the Empire so I was still fairly new to it. My customers showed up and not a single other GM was running a game, so they all joined my session. 34 players sitting at a giant table made of other tables. It was a mess. I think it was mainly enjoyable though because there was essentially 3 groups of players where vets took lead. Both sessions of this were roughly 3-5 hours each though. As it became a headache for everyone playing the longer we played. Combat is really bad unless you do major battles, we did the Battle of Yavin and had a blast fighting along side Yellow Squad.

In the wayback, I ran a group of a dozen+ in a Twilight 2000 game in college.

Roll to hit, roll a few d6's for damage, add it up, roll for hit location, apply damage.

...up to 5 times depending on weapon rate of fire.

...repeat up to 12+ times for your PC group of 12+ (if everyone's shooting, depending on ROF, 30-60 rolls)

...Referee does likewise for the horde needed to combat the squad of PCs.

...for ONE round of combat.

...repeat.

...only help being combat is very deadly, so less actors to roll for next round.

Meal breaks between each session of combat?

I still loved it (back then). Complete original and 2nd edition sets in the basement.

The largest group of people I GMed for is 3. I have had a hard time finding people to play.

I've been asked to DM for the 6-7th graders at work and I was contemplating just having an open admission policy.

The question is, how many are too many?

"And the Lord spake, saying, Four shall be the number thou shalt GM, and the number of the players will be four. Five shalt thou not count, nor either count then three excepting that thou then proceed to four."

"Amen."

I had 7 players in my group and it worked fine, the go to response for advantage was either to trigger a crit, activate blast/autofire, or pass along a boost die to the next player. People stopped and thought about triumphs. Of course I handwaved a lot of rules, only nemesis had Stat blocks, and I eyeballed wound and strain threshold... typically a "solid" hit (about 10 wounds after soak) and a crit would take down a rival in a single shot, or 2 solid hits. And I use wotc minis (often with maps) to keep track of who has and has not gone yet.

I run an Edge group at my FLGS. I had 13 players at the table during one session which was a bit crazy. Luckily one of the players stepped up and is running a second table. We usually have between 5 and 9 players per table depending on how many show up to play. What is fun is that the two tables always start and finish as one group as the adventures I put together are different but start and finish together. I think the biggest group so far has been 17 total, 9 at one table and 7 at the other.