Rule of thumb:
you should have 4 mags for every pistol.
3 of them should be loaded at all times. Every month, rotate the unloaded one, so that every mag gets a month off once every 4th month. Number your mags so you know which one is next for it's month off. Doing this makes the mag springs last longer. Also, when your mags are numbered, you can keep track of failures and see if one particular mag is causing all the problems. If so, Replace it with a new one. And don't give the new one the same number as the one you remove from the rotation. For instance, if you throw out #4, it's replacement should be labeled #4A, #4*, or #4', or some such.
Carry one spare or two spares, it's up to you. You should have a 3rd ready and loaded at all times even if you don't want to carry it. You never know when you will have a good reason to change your mind and carry 2 spares.
This is, as I understand it, the "correct" or ultimate way to maintain mags for SD pistols. If anyone has an alternative methodology, I'd like to know what it is.