It seems that there are always plenty of varying opinions on carrying spare ammunition. Some people say that, since the average gunfight typically consists of 3 or less shots being fired, the ammunition in their 5 or 6 shot revolver, or the ammunition in their 7 to 17 round semi-auto, is more than enough. Others insist that you can never have too much ammunition available on your person and they'll slide a spare magazine or speedloader into every available pocket.
While it's true that most gunfights involve 3 or less shots being fired, in today's complex tactical environment, we are seeing more and more cases where criminals attack in groups of 2, 3, or more, and in some cases, the criminals are dressed in body armor. We can ill afford to rely on statistical probability alone as the determining factor for how much ammunition we may, or may not need. Something I've said to my students before, and I'll repeat here; unless you are going swimming, there is really no such thing as "too much" ammunition. On the other hand, the concept of "too little" ammunition is easily quantifiable.
Like nearly every other piece of equipment we select for our personal protection plan, choosing how much spare ammunition to carry, and in what manner, is a matter of balancing practicality with plausibility. I'd suggest that the primary reason a person might carry at least one spare, fully-loaded magazine for his/her semi-auto is because the gun itself implicitly relies on a properly functioning magazine in order to operate correctly, never mind the fact that having the extra 7 – 17 rounds of ammunition on your person is never a bad thing. On the other hand, the person who chooses to carry a 5 or 6 shot revolver would be well advised to consider whether or not those initial 5 or 6 rounds will be enough in the event of a worse case scenario situation.
Whatever type or caliber gun you choose to carry, be sure to put some serious thought into your possible spare ammunition needs. The middle of a gunfight is not the time or place to be wishing you would have grabbed that spare speedloader or magazine on your way out the door for the evening. Make it a habit, and you will never be without when or if you are in need.
Be Safe!