When it comes to the use of handguns in self-defense, the current fashion is to go for high-capacity 9mm pistols. They're ubiquitous, and for good reason: the recoil is manageable for most shooters, there are plenty of rounds on tap (up to 17 or 18 in many examples), and if a quality hollowpoint round is used, they offer adequate terminal performance. Even compact examples, holding only 6-8 rounds, offer slightly greater power than the traditional .38 Special snub-nose revolver, and greater speed and ease of reloading. All in all, very useful weapons.
However, there's another aspect to it, and that is the age-old topic of "stopping power". We've pointed out before that there's no such thing, at least in theory, because one can't measure in any meaningful way how many rounds are required to stop an attacker. If he's merely out for a quick score, and meets spirited opposition, even one round that misses him might be enough to make him turn around and run for his life. On the other hand, if he's hopped-up on drugs, he might not feel half a dozen or more torso shots, and carry on attacking until his body finally shuts down. I've personally witnessed an assailant who'd been shot multiple times in the chest (including one round that went right through his heart), but he still lived long enough to reach the defender and open his skull with a machete. Both died on the scene. Can one call that a "successful" defense, in that it stopped the criminal attacker, but did not save the life of the defender? I can't.
On the other hand, there's a time-honored and generally accepted rule in defensive shooting that tells us the bigger and heavier a round is, the more likely it is to stop an attacker. It dates back to the days of the Civil War and the Wild West, where "manly" revolvers were in .44 or .45 caliber, while "lesser" pistols were in .36 caliber or even below. In general terms, one or two solid lead slugs to the chest from a .44 or .45 did the job, whereas it often took more of the smaller .36 rounds to achieve the same result.
Modern bullet technology has improved the performance of smaller cartridges like the .38 Special or 9mm, but it has also improved the performance of larger-caliber rounds. In general terms, based on actual street performance, the bigger stuff still stops attackers faster and more effectively than the smaller stuff. Of course, any round that shuts down the attacker's central nervous system is likely to achieve a very quick stop indeed, but that takes a shooter who's fast enough and accurate enough to do that on demand. Most of us aren't that good, so a more powerful impact is a distinct advantage when dealing with a hopped-up or fanatical attacker who has no intention of stopping, no matter what.
(As one measure of that: ask hospital emergency room nurses and surgeons how many shooting victims survive hits - even multiple hits - by 9mm or .38 Special handguns, versus how many live through hits from .40, .44 or .45 weapons. When I was a prison chaplain, the medical staff there told me there were any number of convicts with scars from the smaller cartridges, but very few showing scars from bigger ones. That's because those struck by bigger, harder-hitting bullets survived less often.)
That's been borne out over the past few years by videos of police and defensive shootings all over the world. We see how, time and again, those with smaller-caliber firearms (like 9mm pistols) fire half a dozen to a dozen rounds in order to stop an attacker. One or two rounds just won't do the job, because they're not hitting a vital target. In civil war situations, where attackers roam in mobs and attack in large numbers, the higher magazine capacity of a 9mm is valuable - but only if each round stops one attacker. If you put ten rounds into the first attacker, you may stop him, but then you've used more than half your pistol's magazine and other attackers are still coming towards you. You're in trouble.
Therefore, choose your defensive handguns in the light of what enemy(ies) you may face. If you suspect you may have to drive anywhere near, say, an Antifa or BLM demonstration, you might want to carry large quantities of ammunition with you, but you might also (and, to my mind, should) carry a more powerful weapon than a mere handgun. An AR15 or similar defensive rifle can provide far greater stopping power, and far greater practical accuracy, than a 9mm pistol.
If you have no choice but to rely on a handgun, it's worth relying on one powerful enough to get the job done. Most days I carry a 9mm or .38 Special handgun in a pocket, because it's the most easily concealed weapon, and because I'm in a relatively low-crime environment. If I were in a more progressive-left city with legions of aggrieved activists, something with higher capacity might be needed. However, given the realities of so-called "stopping power", and my experience with it in a civil war and unrest environment over many years in Africa, I'd probably choose something with greater power, like a 10mm, .44 or .45 handgun.
Some smaller "heavy" cartridges offer the advantage that a handgun chambered for them can hold almost as many rounds as a 9mm pistol. Looking at my gun safe, a 9mm Glock 17's magazine holds 17 rounds, whereas a .40 S&W Glock 22 holds 15, and a 10mm S&W M&P also holds 15 (the latter round being considerably more powerful than a .45 ACP, while we're at it). A heavier, harder-hitting cartridge does not have to imply lack of magazine capacity. Even that may not be an issue, depending on one's choice of firearm. A .45 ACP Glock 21 will hold 13 rounds, still a useful number. All of the rounds mentioned in this paragraph will deliver a harder punch than a smaller cartridge, and are likely to cause greater pain and disruption to the person hit by them. There's a lot to be said for that.
So, by all means, if you can only manage the recoil of a 9mm pistol, go with that option. It's not a bad choice, and will serve you well if you put the bullets where they're supposed to go. That takes training and practice. However, if you can handle the recoil of a more powerful round and shoot it accurately, there are good reasons to consider a handgun using them. If I'm visiting a city where crime and other hazards to my health are more likely to be encountered, I'm very likely to pick a large-caliber firearm. (To take just one example, a .38 Special snub-nose revolver can be dropped into the average trouser pocket very easily, but a .44 Special Charter Arms Bulldog is almost as light and not much bigger. Given a pocket big enough to conceal it - and there's nothing stopping us from adding cloth to an internal pocket to enlarge it - it makes a handy choice, and using a round like Buffalo Bore's .44 Special full wadcutter, I have every confidence in its stopping power at close range.)
Thoughts to consider in this violent, criminal day and age . . .
Peter