It depends on what you plan to do with it, but like he said, not much is needed to keep an AR running. They really don't break very often. It is more common to see some of the small parts get lost during routine maintenance and the like, or see the handguards crack while trying to be muscled off the wrong way.
I keep a few small parts on hand for my homebuild M4gery. I keep a spare firing pin (never seen one break either, but hey you never know) and firing pin retainer, extractor and extractor spring, a set of the takedown pin detents and springs and a spare spring and snap ring for the dust cover.
These will cover 99% of any break/loss I've seen.