Would I need 17 rounds? Probably not. These thugs are basically cowards and pick on unarmed people. I would submit that one guy with a revolver could have turned them away. The crowd mentality only works if you're not the guy in front getting shot.
I do find it interesting how when a strategy is used effectively against us by others, we downgrade the users with a derogatory personality trait classification, but when our folks employ similar strategies, they are considered to be smart strategies used by brave people.
It is not cowardice that to attack a target in a manner for which the target doesn't have much chance to respond effectively. That is why groups like SWAT wear full body armor, carry what they expect to be superior fire power, come with vastly superior numbers, and use tactics such as surprise, speed, violence of action, and even the cover of darkness to conduct their operations. There is a very good reason why a SWAT team isn't a single person in spandex bike shorts and a neon shirt armed with a warrant, badge, and a gun.
Risk reduction is a strategy applied successfully across human cultures and the animal kingdom. There is a very good reason top tier carnivores don't primarily prey on each other. The risk is too high. Risk can be reduced by methods such as using superior numbers, better armor, picking weaker targets, using surprise, speed, violence of action, and sometimes using the cover of darkness.
We can call the bad guys cowards because we don't like it that their tactics do work with an amount of success that causes us discomfort to consider, but the fact remains that the tactics and strategies are ones that work, that we used ourselves, and that are designed to reduce risk such that even if you aren't successful, then at least chance of injury is reduced.