You know, for practicality's sake, there's no way to arrive at a definitive "answer" for this sort of question (as has been intimated by more than one person responding).
Trying to focus on sheer weapon capacity can distract someone from the arguably more critical factors that ought to be considered.
Things like the user's - knowledge (laws, tactics, etc); training; physical condition (including any disabilities, injuries or other restrictions); practice regimen; skillset; familiarity with the weapon (manipulation, maintenance, appropriate carry method, etc); experience and mindset.
Time and time again, being able to make accurate and effective
hits on the intended threat targets was the one thing that stood out most in the minds of several cops who have been involved in shooting incidents to whom I've listened.
Not caliber. Not capacity. Not handgun type. Not brand of weapon or ammunition.
Being able to make accurate and effective hits on their attackers.
Shooting incidents are often described as unexpected, dynamic, rapidly evolving and chaotic events. Often occurring in bad lighting conditions. Physically and mentally stressful. Creating a fear-induced hormonal stress response (different than just running around a track or doing pushups and physically taxing the muscles).
Capacity? Sure. Something to think about ... after other critical influences and conditions have been considered.
Properly supervised Sim gun/marking cartridge training, and Force-on-force training (again, properly supervised) can help reveal things about training and skillsets ... and whether or not someone is able to effectively use good tactics when caught by surprise and put under stress ... and perhaps help someone determine how well they can access "ingrained" training, as well as their decision-making abilities and skills under stress.
There's still something to be said for having a solid foundation in a handgun skillset, though, whether it be for semiauto pistol or DA/DAO revolver ... or both (if you're lucky and able to develop and maintain skills using both platforms). You have to have something to build on, after all.
Ammunition capacity is just ... well, available ammunition capacity. Doesn't mean it'll be lawfully, appropriately, safely, accurately or effectively used.
How much capacity is "enough" when it comes to a gas tank in a car, truck, tractor, motorcycle, boat or airplane? Does whatever capacity you think is "enough" going to guarantee the operator is going to be able to complete their trip, not make mistakes, or be able to properly respond to exigencies or emergencies?
I've often thought that a dismaying number of folks nowadays sort of look at having "hi-cap" mags in the same way some folks used to look at having a rabbit's foot on a key chain when I was a youngster ...