O-rings if you use them on the ejector. Ejector springs or a full ejector kit. Spare bolt, cam pin, firing pin. Full set of springs and detents should cover you. Gas rings too. When your bolt gives out completely, it will be a broken lug. IIRC, that can come around 10k.
I took a few friends to Front Sight for a week in the Practical Rifle class. I supplied all their rifles, including a couple that I had milled and built from 80% lowers. One was built into a LH gun with the Stag LH kit. It included a LH upper but all the lower parts were still RH. They are a pretty cheap outfit, apparently, which will be emphasized later. Anyway, I bought separately an ambi- safety and ambi- mag release. This was just to get a rookie through a 4-day, 100-yd practical class, so I just built it with the Stag trigger parts.
On the first afternoon, the Stag trigger went down. Of course, the instructors assumed I screwed it up building it, which in their experience I'm sure is the case 96% of the time. But when I came back from the car with a new trigger in the gun in about 15 minutes, I think they were impressed.
The disconnect hook on the Stag trigger had broken off. In addition to a complete spare rifle in the car, I also had my complete AR parts box. I didn't really purposely buy spare parts (I default primarily to the backup gun principle) but I also organize and keep all the parts I've ever gotten.
Stag would not even reply to my emails to replace their failed part. I guess repeat customers is not a thing they need.
The problem I have when I get too many spare parts on hand they usually turn into another complete AR, then I have to make room in the safe for it and need more mags, and then I need to get more spare parts to replace those that transmogrified into new gun...