This news report was posted on YouTube in December of last year and is the first I heard of the incident. A US Marine home on R&R from Afghanistan was at a movie theater with his wife and was attacked afterwords in the parking lot by a group of teens numbering about twenty. They took him to the ground and beat him, and one of the attackers punched his wife in the face and knocked her out cold.
This sort of attack is one that I find to be the most relevant to where I live. There's not a whole lot of gang activity, at least not like what there is in a major city, but there are large groups of aggressive teens that beat people and rob them. About a year ago a man and his girlfriend were jumped at the local movie theater by a group of Hispanic teens while crossing the street, and they hurt him badly.
Twenty attackers is a huge disparity of force, and constitutes a deadly threat in any scenario I can think of. The chances of getting kicked or stomped in the head or falling and busting your dome piece after getting knocked out is very serious. Such numbers of scumbags may not need double stack capacity to be addressed as they will probably scatter in all directions on the first shot, but the comfort of a having 15+ rounds on station with a couple more magazines in reserve seem to be a good idea.