I tend to buy ten to twelve magazines for each of my centerfire handguns. But for me the minimum number is nine.
I carry three mags total on me when I carry. Two on the belt and one in the gun.
I have my primary load out, and two spare load outs, but if I find a deal that is like half off or less than the retail price, I might buy extras. That's just for the standard capacity mags.
The ten round mags will always be available, so I usually just buy three for each gun.
Certain mags, like polymer mags, out to be considered as being disposable items. You are not going to be able to fix damaged plastic.
However, for my steel bodied mags I have springs, followers and base plates that I keep on hand. At least enough of these parts to fully rebuild half of them. Certain mags are worth repairing and certain ones are not. And that is how I wound up with so many Glock mags.
I have managed to get really good deals on some. I got my Sig's mags at half off. Normally 50 bucks each and I got them for 25 bucks each.
For my wife's P365, I got her three 10 rounders, five 12 rounders, three 15 round mags that have been discontinued and three 17 rounders. So she has a dozen mags. Half of them I bougHt used, but barely used, for 20 bucks a pop.
I have range mags and carry mags. Usually the carry mags are OEM. Except with my 1911's. This all get Wilson combat and Chip McCormick mags.