FBI stats, IIRC, starting four or five years (again, that's what memory says) back put the number of incidents where there is more than one attacker at over 50%. Also, many problems with semi-auto pistols can be attributed to a problem with the magazine, not the pistol. This indicates that an inclination toward some sort of high(er)-capacity semi-auto and at least one reload, more to cover a malfunction than anything else, should be considered.
It may be more useful to think of magazine or ammunition capacity in terms of number of bursts than of number of rounds. I am an advocate of the ability to make accurate/on-target bursts of 3-5 rounds for a number of reasons starting with an article I read (I think) in the 80s that had the title "Double-Tap, Assess, and Die" which opened the debate in my mind about that.
Given a 3 round minimum burst, I look at capacity in terms of how many of those I have. I may not need as many rounds for a given single target, I may need more, but it has been helpful to me to standardize on 3 rounds per in my evaluation of various weapons and capacity. So 8+1 would give me 3 bursts, 15+1 would give me five, etc.
Everybody says handgun rounds are anemic and the only reason they carry handguns is because they are handier and we can't, in most places, carry rifles wherever we go. I believe them. It seems best therefore to not rely on a single hit to stop, even if it does so most of the time, and to make up for the lack of power in the handgun round with rapid multiple shots (it also gives me a better chance of getting a CNS hit, which is what I'm always going after from the get-go).
Given all that, I continue to recommend, where possible, higher over lower on-board ammunition capacity and at least one full reload for the handgun whether it be revolver or semi-auto or a pepperbox. How much more, or less, as always, is up to the individual. Whatever you choose to bring to a class I teach, I'm going to do my best to make you able to use it the way you might need to regardless. I would think that most instructors will.