Honestly, I rarely carry a spare mag. My carry gun holds 14+1, and I've shot it enough to trust that the mags are reliable. A spare mag is better than not having one, but it's also extra stuff to carry.
Kinda how I do it. If it is summer, my gun (which gets shot rather often so I know how it acts and I know whether or not my magazines are reliable) carries 17+1, and I don't carry an extra mag because it is harder to conceal and I've hit my personal annoyance limit on stuff in my pockets etc when I'm wearing shorts and a polo.
If it is winter, I probably have an extra mag weak-side OWB (matter of fact, in the mag pouches I use for IDPA---yay, using actual carry equipment in a match!) since I'm wearing a fleece or something over pants and a polo. Easy to hide, no annoyance factor, I've got 34 rounds on-board, and with that setup I can easily go from empty/locked back to reloaded and firing a shot in less than 2 seconds. (If I've already fired 18 rounds, please come help me because I'm in serious trouble.)
Would I do it differently if the firearm I was carrying held less ammo? Maybe....I dunno. There have been times with a PF9 in dress clothes with no extra mag....I wasn't happy, but that was what I had to work with. If I happen to have a G26 in deep concealment I'm at 10+1 with no extra mag just because deep concealment needs tends to also mean I've got no place to put a mag where I could actually get at it in any reasonable amount of time.
The whole things really depends on your comfort level, how much you trust your equipment, how much practice you have with your equipment, your lifestyle and the need to lots of rounds... (How fast is your reload? After all, if you need it, you need it Right Now as someone else is trying to kill you...)
I know a number of people who pocket-carry a j-frame with 5 rounds, and call it good. I'm not going to tell them they are wrong. After all, stats say a majority of altercations are stopped with 1 round or less (hits or not). If the gun goes bang, that's significantly better than yelling loudly.
I do tend to think there is an outer ridiculous-level limit on gear, based on a realistic appreciate of your lifestyle. People can go beyond that, of course---load yourself up, and have fun! No issues. That, however, is different from saying it is necessary or needed.