I believe 5 magazines per hangun is a good minimum. I have three Glock 19s. So I would have a minimum of 15 magazines for those pistols. My minimum is 10 per rifle. If I have 3 ARs, I would have 30 AR mags available.
However, I also use different mags for training than for carrying. I keep a set of unmarked, clean, and unabused pistol magazines for carrying. I have a set of similar magazines for my rifles.
I treat training magazines as a consumable part. They get worn out, abused, sometimes lost, and possibly broken. So having a number of training magazines available spreads the wear across them and they seem to last longer. Training magazines probably doubles my number from above.
a serious gun guy I know who works LE in Nevada maintains a goal of 10 mags loaded with duty/carry ammo, 10 mags for training, and 10 new in package for each gun.
I have around 100 aluminum GI mags and around 40 PMags. Any and every time I find a deal on them and I have the cash I buy them. I recall the post 94 days well. Never again for me.
I dunno, I guess it's whatever YOU 'feel' is 'right'.
I tend to keep, and carry 3 mags or 4 speedloaders for each. But having more to rotate around, or more on hand in case one needs to be replaced, or parts for those, seems perfectly sound in concept.
I would also say that I would think a guy who competes might well feel the need for more than the 3 I might feel are appropriate.
YOU decide. I personally don't care if ya have 10,000 for each and every gun ya own.