Like Working Man, I find it useful to have several loaded mags ready when I'm shooting (or introducing a new shooter to shooting). If I'm not mistaken, AR-15 mags were initially envisioned as disposable items. They're not indestructible, so it's good to own spares.
A couple of sets of half a dozen to a dozen for each AR makes sense to me. Add as many as you think you'll need if you like the idea of owning all of the mags that you expect to want in your lifetime. That way, you need buy nothing else for it in the future but ammo.
To me, clips are a related issue, so if you'll indulge me:
It's always good to have whole bunch of en bloc clips for your M1. A couple of hundred, aqcuired by buying clipped ammo from the CMP, would not strike me as crazy.
Loading moon clips for revolvers can be tedious. It's not bad with RIMZ plastic moon clips (just broke one this weekend, though). On the other hand, metal moon clips. while also a dream to shoot with, are a pain in the neck to strip and fill. I like to load all of the moon clips that I have beforehand, and I don't have enough. I've got 15 --whoops, 14--RIMZ moon clips and about ten metal ones for a Smith N-frame revolver in .45ACP. That gives me almost enough for three boxes of ammo, and we shot more than that out of the Smith 22-4 this past Saturday. I loaded clips while somebody else shot, and we took turns. That's OK, but I'd like it if that weren't necessary.
People recommend metal moon clips for carry revolvers because they hold the rounds more securely, but I plan to order a bunch more plastic ones from RIMZ as 'range clips,' as well as some metal ones because they're available more cheaply. Ideally, I'll have enough moon clips for several boxes of ammo, anyway.