

shadowdark might work, but the biggest group i ran that for was 7 people and the torchtimer became somewhat short for that amount of people, but instead of a real time clock you could use a 10 round counter. The system itself is simple enough to avoid lengthy rule discussions and keep the players eyes in the game and not on the character sheet. however most adventures seem to be writte n for 3 to 5 people, so thats something to keep in mind.
have you considered running two games, with a gamemaster for each game, that influence each other or share a finale where all players participate? i think that would run better in most systems, simply because downtime between a players turn can get really long with that many people and discussions what to do take way longer with 12 people than 5 or 6.








tbh i think any reasonably simple and fast game could work for your endeavor(the borgs, cairn and so forth), if you accept the inherent challenges of your higher player count. shadowdark just happens to be the game i enjoy to run the most.