I have a confession to make. I'm a magazine hoarder of near clinical proportions. Especially with regard to my battle rifles. They take the most abuse in training and use so I feel compelled to have an overabundance. Even during the big scare I made a tactical upgrade by trading 10 Pmags for 30 C products new in wrapper GI mags. This is in addition to the foot locker of used mags I bought at a sale a few years ago. No such thing as too many.
I do prefer mil contract mags, and the larger rental operations have proven that they are longer lasting and take heat cycles better than plastic mags. The brownells GI mags are supposedly the best price for the functionality. I personally prefer C products contract mags. They handle debris slightly better than most and the outside stays buff for a longer time. Most mags get super slick after many reloads and handling, which can lead to dropped mags. They have the green followers which work great, but they aren't the new fangled tan follower that is proven superior and the military is phasing in the tan follower. Beware the black follower. It's not anti tilt and is the least reliable in adverse conditions. Green is still good to go. Magpul followers are great but costly. You can hedge your bets and decrease your loss rate by having some rebuild kits on had as well.
Pistol mags are a bit more robust due to their smaller size and construction. I have a minimum of 3 mags for carry and 5 for training. That way you aren't loading and unloading your carry mags all the time. I have 20 or so extras per model rolling around just in case. I've streamlined my pistols recently and have dropped the 10mm, 40SW and only stock Glock 9mm mags, HK usp 45 mags and a few pocket gun mags. The LCP being my choice for deep cover carry.
I ruin 2-3 rifle mags a year. I ruin 1 pistol mag every couple years. That's for our entire household. Those 30 mags I bought are a 10 year supply if I keep training as I have with the family. I do my best to replace them as time and resources warrant, but I do keep 50 contract GI mags in reserve just in case. My hoarder status is showing.
You also need to remember that I keep lots of AR15 mags in my Caches and I always keep a thousand plus loaded in my bastard child mags and keep them locked away in my safe. No such thing as too many! My lightweight plates stay loaded all the time as well. 3 25 round 308 mags and 3 glock 17 rounders with plus 2's. That's a lot of firepower in a hurry.
Specialty Mags- I do have several rifles that are AR10 or long range in nature. My 300 Win Mag competition gun has 3 magazines for it. They are little 3 round units, and I bought them mainly due to the fact that you can use them as a single shot follower in competition more readily than the stock Remington bottom metal. Not something you need to stock deep. Same goes for the rest of my long range stuff. I do have a Beta C Mag, but I've honestly never used it. I see no need. You'd have a useless slag of metal for a barrel after 100 rounds of sustained fire. I consider it an investment.
My AR10's on the other hand, each have 21 mags for them. I don't shoot them enough or train with them enough to warrant the extra mags. The 20" gun uses 20 rounders and the 16" uses 25 round Pmags. Luckily Pmags fit the LWRCI platform. They are easy to find and plentiful again. The AR10's have a definitive use and plan of action for my defense plan. They don't need all that many mags due to this. They aren't something I'm going to go patrolling with, just too heavy. It's best for killing buicks and turning cover into concealment.