Not counting a special Encounters adventure*, the largest I did was 11 for a one shot adventure. Large groups do not do well, at least not with high schoolers. One PC was OP, defeating the entire first encounter by himself then bragging about how weak everyone else was. This lead to the other group attempting to "put him in his place," which became a massive fight. Yes the first PC was OP, since he killed everyone else before dying of his wounds... leading to a TPK without a single monster ever making an attack roll.
I had a DM who regularly did 9, but that was an odd group broken up into three overall cliques (3 Roleplayers, 3 Rollplayers, and 3 Watchers). My group (Roleplayers) engaged the story element of the game while the Rollplayers read, played on the gameboy, whatever. When combat began, the Rollplayers perked up and dominated the spotlight, while the Roleplayers just sighed and tried to get past it as quick as possible. The watchers just sat back, periodically interjecting into the story and combat.
* Up to 6 groups of 5 would enter the dungeon complex from different areas, each with their own DM. After 4 hours, each group that made it through their section would arrive for the final battle in the center of the complex run by a primary DM and up to 2 assitant DMs (I had one). In theory the final battle could include up to 30 PCs, but our story only had 19 players with 2 PC deaths, leaving 17 to DM.