If your plot really does require throwing 12 people in a room together, and most or all of them must actually talk at some point, don't try to write out the literal sequence of dialog word-for-word. Instead, give the reader a high-level summary of (at most) one to two paragraphs describing the conversation as a whole, and its result. If it's just "everyone brought character X up to speed on event Y," then this can be a single sentence.
For example, you might write:
With the twelve of them reunited, the bickering started almost immediately. Susan began throwing around her usual paranoid accusations in every conceivable direction. John responded with his trademark sarcasm, and the conversation deteriorated from there. Jane sank to the floor, head in her hands, trying to blot it all out.
(It is assumed, of course, that John, Susan, Jane, and the other nine characters all have established personalities that your reader will be familiar with. Otherwise, this would make very little sense.)
This sort of thing should be rare. Most of the time, you don't want 12 characters in a scene together. It's too hard for the writer to manage and also too hard for the reader to follow. To some extent, you can get away with it if there's some additional structure to help the audience keep up with the plot. For example, in Ocean's Eleven, when Danny describes the plan, he's doing most of the talking, and it almost feels like diegetic narration rather than dialog. If the film had instead tried to portray a scene of all the characters coming up with a plan collaboratively and organically, it would have been much harder to understand.