I am a member of a sci-fi gaming club that back in the day had Star Trek gaming sessions with over 20 people playing. it was usually utter chaos as there was only one GM.
Then we started playing Mechwarrior (again with 12+ people per session), but we wised up a bit and had 2 GMs, of which I was one. The 2 GM style worked very, very well for us because myself and the other GM were on the same page on style, tone, plot, etc. Usually we each took one of two distinct groups of PCs, had an idea of where things were going, conferred every once and a while about what the sub-groups were up to, and winged it from there. It was particularly entertaining when one group did something that caught the attention of the other.
On one memorable occasion one group of PCs was on night patrol and stumbled across an approaching raiding force sneaking up to the town they were defending while the other group was engrossed in a poker game in the town about to be attacked. The nice thing was that each group really didn't know what the other was up to most of the time, so the first indication that this wasn't going to be a light "social" session for the poker players was the Mech firefight that broke out around them. It was fun to describe the elemental forces of the huge energy weapons from the ground level perception of pilots who normally see the action from a Mech cockpit, all the while desperately trying to make it to their base across town to strap in and join the fight!
It takes some work and communication, but a 2 GM solution to large groups is a viable one in my experience.
TGA