Should always carry at least 2 additional mags, or have a partner with you who is also armed.
Reason?
o If you have a FTF, drop the current mag and insert a new one.
o If you are wounded in the arm, you'll have to use one hand to reload so you should carry a mag next to your gun.
o WHEN the fight is over, you're standing there with an empty gun what do next? "Duh", anyone?
Chances of a FTF and an empty gun if you have an 8 round single stack - very high. Assume you hit the target with at most 2 of 5 rounds.
So at least 2 mags, one next to the HG, and don't attempt to fight with mags in your pockets. They are for resupplying your belt carried mags. Thus carry 1-2 more in your cargo pocket.
The number of absolute rounds in your gun has NO bearing on how many mags you are carrying because a magazine can and does go bad, jams, falls apart - they're just thin metal and plastic things.
$.02