I own enough to keep loaded for the range that I could get through about 3 trips without reloading any and about an equal number to that un loaded, brand new, ready to rotate in for trashed ones. For most everything else I own it's enough loaded to get through most or all of one range trip without reloading plus a couple unopened spares but I stack ar ones a bit deeper. I  have slowed down buying them but not stopped. My Glock pattern ones are the next biggest stack. Then 9mm and 45cal 1911. When I own multiple guns of a given platform that feed from one kind those mag piles tend to get bigger.

My safe is somewhat diverse but I have with new purchases gravitated to a smaller number of common platforms specifically so I can more cheaply accumulate substantial magazine stashes. So nice to take a gun home with a crate of loaded range magazines sitting there waiting for it and just toss the one or two that come with it into the spare pile.

Well said indeed.

infantry loadout for fighting guns seems to be a good bare minimum.

Triple digits is not unreasonable

Having been well established in the shooting hobby when the 1986 ban on production of full auto for civilians, the 1989 ban on importation of fun guns, the 1994 ban on high capacity magazines, and so forth ... the answer to "how many mags" is ALWAYS more than you think you need.  

Magazines are wear items, expendable and do get lost or damaged.

Since AR15 magazines regularly go on sale for $8 to $9 I recommend buying 10 at a time when you find a deal.  Once you hit 50 mags evaluate your other rifle supplies (ammo ... which will be harder to regulate than mags because 223 is a hunting caliber), quality of light / optic / sling.  

For a person on a strict budget this is my "go to advice" ....

FOR EACH AR15 YOU OWN (THIS IS YOUR MINIMUM):

7 training mags
14 primary mags (this is two complete "load outs")
2,500 rounds of training ammo (replenish when you hit 1000 rounds)
1,000 rounds of defensive ammo
1 quality sling (example: BFG)
1 quality weapon mounted light (example: SureFire 600)
1 set of quality back-up sights (example: Troy, MBUS PRO, KAC)
1 quality optic (example: Aimpoint, ACOG)
1 quality cleaning kit (example: OTIS)

FOR EVERY THREE AR15 YOU OWN:

1 spare action spring
3 spare firing pin retainers
2 spare complete bolt assemblies
2 spare extractor with spring and o-ring
1 spare fire control assembly (2 pins, hammer, spring, trigger, spring, disco, spring)
2 spare charging handles
1 set buffers (carbine, H, H1, H2)
1 spare magazine release assembly
1 spare bolt catch assembly
2 spare buffer retainers with spring
1 spare castle nut

I started assembling AR15s in 1984 and the above is cumulative knowledge from 39 years of doing so.  I shoot .. A LOT ... as in 2 to 3 times a week and as much as 20,000 rounds a year.  If you do the above ... AS A MIN ... and start with a quality AR15 (like a BCM, Colt, Daniel Defense, KAC, LMT) you'll be set up well.

ETA:

Your target should be a minimum of 50 mags per rifle.  At $8 each that is only $400.  This is less than most of us spend on optics.

Don't waste money on upgrading your trigger until your mag supply is 50 (min) and your defensive ammo supply is 5,000 (min).  Then drop $100-200 on a better trigger.

You can never have enough

Botach has ETS AR mags 30 Rd $4.50.  
AR-15 com has SHK S Korea 30 Steel mags $6.99
These run well in all mine .

So do I.  

I was going through my stash a few days ago and I have enough to last me 200 years based on my range mag life and reliability

I have been using and an abusing the same 16 mags for years.

Same. When a mag is $100 you’ll regret not buying more.  I still have a few mags made in the 70s and they still work.  

Read an after action report of one of the lost platoon members during the battle in the La Drang valley.  He stated that he had 30 mags and did not run out of ammo, those would have been 20 rounders.  He also said he had no issues with his m-16.

Good enough for me, I’ll never be in that situation.

I have 10 CASES that I haven’t even opened. Lol

I’d say 7 and enough non range ammo to fill them plus 1 in chamber. So 7 mags and 211 rounds bare minimum for someone just starting.

If you're going to shoot 100s of rounds at one time out, why reload out in the field? Load mags on the couch while watching a movie.

Some a-hole stole a bunch of my mags out of a vehicle during the ban when you couldn't just buy more cheap. Still pisses me off to think about it. So figure how many you plan to use at once and double it, at least. These are the good old days for buying "high capacity" magazines, right now.

Maybe 2 mags is ok for some people. Probably not for someone who asks how many is enough though. I've been buying when there's a good deal for 20 years and ordered more Okay/NHMTG 30s from RRA a few minutes ago. https://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_ID=2180

100+

They can fail.  And they are often the subject of high regulation.

Mags are expendable and easily damaged. 10 -10 round mags for the range. 20-20 round mags for rifles in the case.
30 Magpul and 30 usgi aluminum mags are a good base to build on.

Like anything, it depends upon your situation, but the more the better. I probably have 50 or so that I functionally checked and then sealed back up for storage. Every time I see a good deal (usually on lots of 10), I grab them. I plan to have an ample stash in anticipation of the day when 30+ round mags can no longer be had, either by SHTF or government bans.

With all the unknown scum invading the southern border, our 10/7 is most certainly coming. While our optimal plan is to hunker down, we keep our stock stored for grab-n-go if it ever comes to that. Same with food.

Whatever the scenario, I don't see the down side to having an excess of guns, mags and ammo, as long as you have the capability to secure them, and to store them properly to keep it all from degrading.

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