For the sake of discussion, lets point out some assumptions that are being made.
1. Multi-perp self-defense situation means nearly a squad (13 for those civilian types, and yes, these bad guys in particular are heavily armed)
2. Since they are heavily armed (as you stated), we are to assume that they chose big bore rounds and each carry in excess of 37 rounds. (12+1 and two mags since this seems to be the consensus minimum in this thread)
3. They are hard core criminals because this entire squad is looking to kill you. (Why would they bring over 450 rounds of ammunition and a small arsenal if they were bothering you to play a game of checkers?)
First things first. I agree with you, more ammo is better if at all possible in this situation. Problem is, chances are you won't have a chance to use it while you huddle behind that dumpster and a hailstorm of lead keeps you effectively pinned.
This would be a prime example that would favor the sound logic that was already presented by earlier posts. Summarized as follows;
1. Situational awareness is vital
2. Not putting yourself in a bad spot to begin with is also key
3. Being proficient with your weapon beats spraying lead
4. Strategy, tactics, and knowledge = survival
And my own opinion... good running shoes +1
Moral of the story is we can always assume the worst, but this doesn't necessarily mean we are improving our odds. i.e. 7 vs 1, 20 vs 1, 150000 vs 1. You are still only one person with a handgun in this scenario. Extra ammunition doesn't make you a better shot, better under stress, or a smarter shooter. I would put my money on the guy with a 5 shot snub, a reload, and a good pair of running shoes. This tells me that he has thought things out and has a plan of action and might just know how to use his gear, or at least one could only hope. We could mold the scenario around your argument for extra ammunition, but that is unfair to the readers and ultimately up to interpretation. There are many people that are not only capable of fending off two assailants with a 5 shot revolver, to many this isn't unreasonable. Paranoia isn't a substitute for rational thinking.