Okay, you have decided to arm yourself with a handgun for self defense, and have obtained a permit if needed. You have purchased and possess one legally - good for you. You have decided on how you will carry it. It is loaded at the ready, on your person, in a holster. You know how to shoot; and are even pretty good at it. So you think you are ready for just about anything. Well if you are not carrying spare ammunition with you, you are quite possibly dead wrong.
There are plenty of gun folks out there who say you don't need extra ammunition. They are usually the macho, all knowing types, the gun gurus. They believe that if you cannot hit that at which are shooting with whatever ammo you have in your gun at any given moment, you may as well forget about having a gun in the first place. That is pretty much abject nonsense. The truth of the matter is that you should always carry spare ammunition if you are carrying a handgun for self defense. There are many reasons for this, some of them being: the probability of multiple attackers in any threatening situation, the possibility of of the bad guy taking multiple hits (even many hits) and the threat not stopping, the chances of having a weapon malfunction that requires you to tap, rack and reload (with a semi-auto), the possibility of you dropping the ammo in your weapon by mistake, and the good old pucker factor making it probable that you will miss more times than you hit your target. Yes folks that good old sphincter muscle tightening itself up, when you are literally just about scared shitless, in order to prevent dirty draws can make you miss. So yes you should carry extra or spare ammunition with you whenever you carry your handgun.
If you agree to that last line above, then it begs the questions: how much spare ammunition should I carry, and how should I carry it. The bare minimum amount of spare ammunition to carry is a practical full reload for each handgun you are carrying. That is a pretty simple formula, but I go beyond that. For example, I usually carry my issued Glock model 19. Fully loaded (as is the way I am required by my job to carry it) it holds 16 rounds of ammunition. By the bare minimum rule of spare ammo, I would then carry a single spare magazine. That is what most of my colleagues at work will do during a normal day. When they go out on a high risk operation such as execution of an arrest warrant they usually carry more spares. I, on the other hand, always carry at least two spare magazines. Why? Well for the reasons I mentioned above, and because if I ever have to reload in a defense situation, I may drop a mag as I try to reload and not be able to retrieve it. Remember that such a situation is one fraught with intensely high stress. If you are using your pistol or revolver to legally defend yourself, your are being threatened with serious bodily injury or death. That is some scary stuff, and when you are scared you maybe shaking or trembling, may start blubbering or crying, may wet your pants, may dirty your draws, and low and behold may even drop your spare ammo while trying to reload. All this stuff not only can happen, it has happened to people in life threatening situations before, and will happen again. That is why I carry at least one beyond a single spare magazine. Sometimes, on certain assignments, I have carried up to 5 or 6 spare magazines, although I think that might be a bit much, let's say overkill, for day to day carry for CCW. I do think though that carrying two spare reloads for your handgun is prudent in everyday carry situations.
That said, you should now be wondering how to carry your spare ammunition. I can tell you one way not to carry it. Do not carry it loose in a pocket. You might think this sounds like ridiculous advice, and you may wonder who on earth would carry like that, but the fact is that too many people do it. It is common among folks who carry revolvers, yet it is a killer, usually of you if you need to reload quickly. If you carry a revolver, I think you should carry spare ammunition in speed loaders, and that the speed loaders should be carried in a speed loader pouch on your belt, and that the puch should have a flap with snap closure over each speedloader. Some people carry in a purse, or in a pocket so as to better conceal a firearm. Then they tend to carry their ammo in the same manner. If you read my earlier piece about holsters, you know I shun such carry methods for handguns; now I am telling you I shun such carry methods for spare ammunition too. If you choose to carry spare revolver ammunition in other than a speed loader, I would suggest at least carry it in speed strips kept in a pouch, in a 2x2x2 speed pouch (not a drop pouch), or in ammo loops - all attached to your belt and easily accessible by either hand keeping in mind that combat reloading is usually done with the strong hand. Don't wear a belt when carrying - your problem - not mine. I suggest you change how you carry for reasons I explained in my post on holsters. Remember though, speed loaders would my my preferred method to reload a revolver.
As for semi-autos, I carry at least 2 spare magazines, each full to capacity. I suggest you do the same, and I highly recommend you carry the spare magazines in a double magazine pouch on your belt. Wherever you carry the mag pouch on your belt (or for that matter wherever you carry the magazines even if off of your belt), the magazines should be accessible with either hand in the even a one handed reload becomes necessary. I used to carry on my weak side, opposite the side where I kept my pistol. I now carry the magazine pouch on the same side, but have since seen the light of others more experienced than I; and I now carry it on that side but on the front of my belt so I can easily reach it with either hand. Sure this means wearing a very loose fitting shirt, a jacket, or a vest to cover it all up, and that is exactly what I do. Remember the gun in a holster on your strong side makes it easy to draw your firearm, and keeps it fairly secure. Same for the magazine if kept as I have described, easier from which to draw a magazine, and somewhat easy to secure.
The type of magazine pouch one carries is another issue. As I said I prefer a double mag pouch. Of course you can use two singles, but I just prefer the double. I want mine to have a positive locking/retention device to maintain my mags inside the pouch with less chance of them falling out. I do not want the to be held in by friction only, so I do not want to use an open topped pouch. What I want is the opening to be at the top (I never wear my mag pouches so that the opening is to the front as you see some do), this way gravity helps keep them in the pouch. I want the mag pouch itself to have individual flap coverings to cover each mag while in the pouch; and I want to to have snap closures - not Velcro. As far as I am concerned, Velcro makes much too much noise, and also gathers much to much lint. In addition I require that any mag pouch I will carry will allow for both mags to be inserted into it in the same direction. What I mean here is that I want to have each magazine available to my reloading hand in the same manner, so that I reload in the same manner each time I reload. In other words, I keep both magazines bottoms up, and so that their fronts are facing the forward part of my body as they ride in the pouch (bullets in the mag pointing forward more or less). You could also keep their back facing the forward part of your body, or could align them left or right, it does not matter all that much in which direction they are, so long as each of them is aligned in the same direction. This way you never need to figure out whether or not you have to turn your mag around before you go to place it into the magazine well on the pistol; or if you have to move your hand in another manner to take hold of this mag or that mag. You wind up reloading consistently each time you reload, and that adds to your speed. When I grab my forward facing magazine, I make sure my index finger rides the front of the mag as I bring it up to place it into the mag well. This makes for not having to look at the pistol and mag to make sure they marry up correctly, therefore you can keep your eyes on the bad guy. It requires some practice, but that pointing finger, pointing the way to the mag well, makes it quicker and easier to accomplish a reload.
Basically that is about it. You have a decision to make about whether or not you carry spare ammo, and how to carry it. My recommendations are above, and the recommendations of others can be found on various blogs and gun forums on the Internet, in gun books and magazines, and at training courses. Whichever way you decide to carry both your firearm and your spare ammo, I suggest you try to be as consistent as possible from day to day. This way you never go to draw your pistol or mag from a place on your belt, and then have to remember oh darn, it is in my pocket, or it is in my bag, or it is in a shoulder holster.
All the best,