Mission drives equipment. What are we trying to accomplish? For starters take a look at Tamara's article in Recoil...
http://www.recoilweb.com/oh-spare-me...ad-132740.html. It is really geared toward CCW and EDC but my quote early in that article is applicable here.....if you empty a 30 rd center fire rifle mag in civilian world USA you're gonna be on the news...if you empty TWO you're gonna be in the encyclopedia.
Look at it this way....4 AR mags loaded with 28 rounds each would allow you to engage 4 people with 28 rounds each....or 28 people...with 4 rounds each....or allows you to engage 14 people with 8 rounds each..... plus whatever pistol mags you have. Doing the math often changes the perspective.
Is that something you REALLY expect to need to do? I'm not saying that CAN'T happen I'm simply saying you should be asking if that is likely in your particular situation. You can roll with 3 mags on your plate carrier and 2 in the gun with a mag coupler and now have 28 x 5 (28 people 5 rounds each or 14 people 10 rounds each) plus whatever pistol mags and probably have far more ammo than you will ever shoot barring the straight up end of civilization and end of the world as we know it scenarios. To be honest, a 20 rd mag in your rifle is unlikely to get emptied so 4 or 5 30rd mags will probably be fine for everything short of global collapse.
At home for HD I generally roll with the rifle loaded with a 30 or 40 round mag. If I have to deal with the extremely unlikely late night home invasion I'm probably gonna only have time to grab the gun the soft armor and hopefully my shoes and get to cover. And a reload is really unlikely to be needed if I have that many rounds on tap to begin with....and besides ...people hit with rifle rounds tend to stop doing bad things without having to be told very many times. So 30 to 40 is probably about 25 to 35 more than I'd ever need.
Now I generally travel a little heavier since all I will have until I get home is probably what I will have brought with me. My current typical travel setup is rifle (Steyr AUG) and five 28 rd mags (4 in a shoulder bag 1 in the gun ) , pistol and 3 spare mags (2 on me and one spare in the shoulder bag) which gives me generally 140 total rounds of 5.56 and 68 total rounds of 9mm. That is not because I think I'm going to need all that in any particular 5 minute firefight...it is because while I'm out of town something could happen (earthquake, hurricane, civil unrest,terror event, Indian uprising , pestilence, etc) that makes return travel home difficult and I prefer not to be ONLY armed with a pistol if things are "open ended" and I might be living on the road for a few days. So that IS essentially my "end of world" load out in case it happens when I'm away from home. Remember the story about Walt Rauch and the group of famous gun writers trapped at Thunder Ranch the week of 9/11 when air travel was halted and they all to a man were carrying J frames and 1 reload or a SW 3913 and 1 spare mag only? They all had to get rental cars and drive hundreds of miles back home with just their tiny "comfortable to carry sucks balls to fight with" pistols and a handful of spare rounds each. That same week my wife and I were on our honeymoon with 2 pistols, 3 mags for each and an AR with 6 mags. Guess who was not panicking about needing ammo..... but again...that is 5 or 6 rifle mags not 15 or 16.
You can certainly roll like an SF team doing LRRP with 14 mags on you if you want, but you also sacrifice mobility and any look of "normalcy". A slick plate carrier under a pullover and a shoulder bag for your mags will be "overlooked" by many. Or that plate carrier with 3 mags ONLY can be "hidden" under a big jacket . I just can't see very many scenarios where other people seeing your plate carrier would be better than them not seeing it. Stealth is your friend as much or more than armor is if you don't HAVE to confront people. And hitting what you shoot at goes a long way toward mitigating the need for a lot of extra ammo.
Edited to ad..... TGS hit a lot of the same points about stealth while I was still typing out my lengthy post. And edited to "revise and extend" the original remarks.