Get ready for a lot of divergent answers on this one.
It is true, and I will eternally concede the point, we have no idea what the heck is going to happen to us.
It is true we could have more than one opponent.
It is true I have never been in a firefight.
It is true a lot of "experts" have never been in a firefight either.
However, we know a lot of general things tend to creep up. Most defensive shootings occur at extremely short distances, in bad lightning conditions, and are over in five shots or less.
Most. I repeat. Most.
Gabe Suarez says in one of his books that if 80% of shootings occur within that short distance, then over 80% of your training should focus on that distance as well. Why? It's the most likely and most deadly scenario with the fewest "outs".
The FBI reports are where the five or six shot figure comes from, and it's also from them that we learn that people who fire high volumes of ammunition are generally ineffective in a gunfight because the reason they fire so much is they just can't get a good shot. If we know most fights are over that quick, then we should focus less on capacity and more on getting the shot on target.
The way I figure it, even if I am in some scenario constructed for the benefit of a 15+ capacity handgun, if there's multiple opponents, it doesn't matter if they're unarmed because we know from the Tueller drill that people can close the gap awful quick. Cardboard doesn't move, and it doesn't fire back.
Ever played paintball or laser tag? Sure sometimes you can come out on top, but if you don't make those first few crucial shots, you're not going to.
If it's only the first few shots that really matter regardless of situation... see where I'm going with this?
That said, it makes some people feel better. I don't mean that in a belittling way at all. I carry .40 S&W instead of 9x19 because it makes me feel better.
To me, factors like:
#1. Do I trust this gun?
#2. Will I carry this gun?
#3. Will I practice with this gun?
#4. Do I feel confident with this gun?
All trump capacity. If the only thing I felt like I could use under stress was a J frame with a 5 shot capacity, that's what I'd carry.
Now it has been pointed out to me that I carry what some consider a high capacity magazine with its 12 round capacity. Well okay it's a little bit of a perk, but to me it's just a slight bonus and not some huge deal. It could hold 6 or 7 and I'd feel fine about it.
It's that old saying about revolvers... "If you can't get it done in the first six, what makes you think another six are going to make a difference?"
My personal threshold... 5. I want five. Must have five.