I've been to a few of Debbie's GG parties and she regularly gets 25-30 blokes coming. Typically five or six girls.
Done in two sessions: first one is a mass wank over the girls faces, then a fifteen minute break, followed by each girl having her own gang bang. The problem is in holding an erection long enough to fuck more than one girl. You fuck one, then move on to the queue for the next girl, take your turn to fuck her, then move on again.