Anything worth shooting once is worth shooting 7 more times.
The fact of the matter is pistol calibers are not great at "stopping". There's plenty of evidence out there where bad guys were zipped several times and although they died from these wounds, it didn't stop them from continuing to kill others. Hell, look at some of the stories of posthumous Medal of Honor winners; guys holding their guts in with one hand and killing the enemy with the other.
Stopping power, from pistols, is a game of chance. Your chances increase with larger calibers and increase with filling whatever with more holes. In the heat of the moment when your life is in danger, dump the mag.
Anyone know anyone who hunts? What does a deer weigh, 150lbs? Ask your hunting buddy about shooting a deer with a .308 and he will probably tell you stories of the deer running an additional 50-100 yards before collapsing. A .308 shot out of a rifle is a devastating round and is capable of killing anything you can find in North America, man or beast. But still, it doesn't always "stop" a 150lb deer. Now imagine your puny 9mm FMJ being tasked with stopping a 250lb bad guy who may or may not be on drugs. Yup, it's probably going to take a few holes to do the job...unless you get lucky.
Handguns, contrary to Hollywood and video games, are really hard to shoot well. The most popular handguns, Glock variants, are really POS. Terrible sights, weird grip, and one of the worst triggers you can get. Guess what; this pistol is the standard issue -or- on the approved list of every police dept in the US (has 65% market share). Getting to the point of being proficient with this gun takes a lot of practice...and training...which most police departments do not do. If you took a random sample of a police force from a large city and pitted them against a random sample of NRA members in a shooting range face-off; the cops would get smoked. Many cops don't even know what the parts are in their gun. "Highly trained" police force is a myth. [many LEOs are 'gun guys' and do a lot of training on their own with their own $$$...just like the rest of the great unwashed...and are highly effective with their weapon. These are exceptions and not the rule]
Aiming for the head, especially on a moving target, is probably not the best idea nor a reasonable request for a typical LEO. That's a recipe for shooting whatever is behind the bad guy. Further complicating matters is the training both LEOs and civilians get when taking a pistol training course; you're instructed to focus on the front sight of the gun and aim for center mass. This is sort of a lowest common denominator style of training for making most people effective with this weapon. Certain pistoleros do better focusing on the target...some guys don't even need sights to shoot an apple off your head...but these are the exception and not the rule. The rule is aim at center mass and shoot until the bad guy stops moving.
Most shootings take place within 6' or so. Past that, with adrenaline pumping and maybe too much coffee, your shots are going to be everywhere. It's why you fire more than once and why magazines have more than 3 bullets in them. You shoot until you see whatever it was that was putting your life in danger stop moving. Then...call a lawyer.
In the event, God forbid, I am forced to shoot someone in order to protect my life; the last thing going thru my mind will be legal/moral ramifications or how many times I should shoot. Shoot 1st & shoot often until the threat is eliminated.
Shooting to "wound" or "incapacitate" is a farce with a handgun. It takes too much precision and is unreasonable in a situation where instinct/reflexes/training is called upon. 100yds away with a rifle? Child's play. Rifles, IMO, are easy to shoot...especially ARs. Those are precision instruments. Handguns are a defensive tool and most handguns carried by LEOs and CCW's are not of the precision variety.