The answer to the title question is the same answer as the one to the following question:
“Deep in your heart, how many do you want”?
Ok, ok. All kidding aside, there is a minimum number and a maximum number. The minimum I would suggest is 8-12. This number includes enough duty mags for your average carrier at work, enough emergency mags for your home defense/always ready to go carrier, and a few extra backup and practice mags for rotation and training. Ideally I would want more than this, and do own more than this. The maximum number would be dictated by both your budget, and at the same time how many Magpul can produce.
While I have never had a problem with a Pmag, I do wholeheartedly recommend testing every mag you own for reliable function. Skipping important steps in equipment procurement and maintenance is only asking for trouble and you are setting yourself up for failure down the road. I would hate to see someone get injured because some goodguy trusted his life and the lives of his buddies to a mag he had just purchased and never tested. Murphy’s Law would state that there is a possibility that some other guy thought he would be funny and had just returned said “brand new” magazine to the store with a used, rusty, recoil spring inside of it, just waiting to fail at any moment. People, please disassemble, inspect, and test! Obviously, a few magfulls of ammo should be put through every mag you own before you consider it for carry or any other “life support” role. I would then set these aside and carry them, but don’t train with them. I say this for one important reason: I see a LOT of people afraid to drop their mags during training because they are either their only set, or are their actual carry mags. “Well, in real life I would let an empty mag drop to the ground during a slide-lock reload”, and their they go developing training scars as they build the muscle memory for placing their shinny new mag into their pocket, instead of letting the empty thing fall to the ground and getting ammo back into the gun to keep it in the fight. Get a set of training mags people! You own it to yourself, your training, and the lives that depend on you. Run them into the ground until they fall apart. Test your gear until you can trust it or it falls to peices and you learn its failure point.
I would like to add to that the part up there where we were talking about dropping mags: I understand that certain training doctrines require retention of empty mags (field environments, etc), and that this part of the article may be considered very “civilian carry” specific. But please note my opinion on that: I say that when you are alone, or the only one shooting at the bad guy and milliseconds count, and you run the gun to slide-lock and still need more rounds, drop the mag and pick it up later! The Commanding Officer should forgive you for dropping the beat up old GI spec mag and choosing so save lives…