Instead of throwing out an arbitrary number (42 would be the answer if that were the case), let's do it systematically.
Why do we need magazines? For practice and for defense. While the two types of magazines may not need to be mutually exclusive, it's probably a good idea to have some range mags around. They get beaten up, used, dropped in the sand and left in a range bag. While good mags can stand up to lots of abuse, you don't want to have to go digging through your range bag to get some nappy old mags to survive TEOTWAWKI with. At least I don't.
Practice Mags
People's practice takes different forms, and with that, you'll need additional magazines. You certainly need to have more mags for say a Magpul Dynamics class than a lazy day of plinking. I'm usually fine with having three or four magazines at the range, but I don't shoot through high volumes during my practice sessions. Your mileage will vary on this one widely based on your training habits, but I suspect the number of magazines will be less than half a dozen per gun for most of us. Have enough extra magazines on hand for a typical range visit. If you're a cheap-wad, you can try to save money here, as a jam or misfeed becomes a training lesson, but I've found that it never pays to skimp on mags. I practice with the same kind of mags that I have for fighting.
Defense Mags
Like your defensive ammo, this should be based off how many magazines you carry in your combat load/fighting gear. I would feel comfortable with having three full sets of magazines on hand for defense. This will put most at between 12 and 24 dedicated defense mags for a fighting rifle and 9 for pistol magazines (two backups + one in the gun seems to be the most common load). These must be high quality, combat-ready mags, not cheap POS knock-offs. They should also be tested for fit, function and reliability.
Why three sets? Well, you have your go-to mags; I keep mine loaded at all times and check/rotate them every so often. Then you have a complete set of backups in case anything happens to the first, and then a final, third set of backups.
Why do you need backups, especially if you train to stow empties/partially depleted mags? Stuff happens to magazines; they're the most vulnerable part of most any semi-automatic firearm. They break, crack and otherwise fail. And in the heat of a gun fight, I can guarantee that at least some will be dumped on the ground. If you don't have unlimited time afterwards to search/scavenge the battlefield and instead have to make a hasty retreat, those mags will be lost.
Why not more than three sets? You certainly could have more than three sets, and if you can get reliable magazines for very cheaply, this is something you might want to consider. But I don't think it's a necessity--there are a lot of other things that take priority over a crate full of magazines. Three sets of combat magazines, plus a set of practice magazines is a real cost--$200 additional per gun, if you're lucky, more if you're not (nine H&K pistol mags...shudder)--and unless you're swimming in spare cash, I think there are probably better things to spend your limited survival dollars on. I'd make an exception if you have supply caches or a dedicated bug-out-land, though--those would need to be stocked as well.
Summary
- Have enough range magazines for your typical practice session
- Have three sets of magazines for your combat load
