I keep enough parts to basically build another rifle. This is a little bit of overkill. Several places sell "oops kits" containing detents/springs, buffer retainer, hammer spring, and firing pin retainer. I'd have at the very least a spare extractor, ejector, their specific springs/retainers, and firing pin in addition to the "oops" stuff.
We have several AR type rifles so I keep complete bolts, carriers, lower parts kits, stocks, and all the tools required to replace/rebuild the guns.
Having a complete spare rifle is great if it's something you can manage. Remember two is one and one is none.