I see this come up a lot, actually.
A lot of guys claim you need dozens of magazines per rifle, however, when are you really going to need 100 AR-15 magazines?
I can see if you plan on selling them in the future in the event of another Federal Assault Weapons Ban, but for personal use between 10-20 will basically last you forever.
Most people who own AR-15s aren't conducting military operations, however, if you ever need to "bug out" and you decide to take your AR-15, having between 10-20 magazines is more than enough to take with you, wherever you go. When you see guys saying you need 50 magazines minimum for these "bug out" type situations, they clearly haven't taken into consideration the survival aspect of the scenario. You're going to need supplies other than your rifle in order to survive. A backpack full of loaded mags is about as useful to you in a situation where you need to be self sufficient, as having a car in the Amazonian rain forest.
In other words, have enough magazines that will be enough for you to go to the range, and carry with you. Have a couple extra in case you break one.
I would suggest buying a bunch of new springs, floor plates, and followers, though. If you use your magazines enough, they may wear out after a while. How long is that? Well, probably a very long time, but there's something to said for the prepared person.
Im not, however, trying to say you can't have all the magazines in the world. For all I care, go out and buy 1,000. It just doesn't make much sense to me if you actually plan on being realistic.
EDIT: I also think that if you ever get into a situation where you need more than 1 magazine to stop a threat (whether that be a person, animal, or cybernetic organism), you're likely not going to be alive long enough to shoot all of your ammo.