I have enough to rebuild every 9mm model except frame and slide, and then lots of extra springs. Oh yeah and spring cups lol. Always have some extra spring cups if you tear your pistol down all the way down. These parts have been useful to me for maintenance as well as trouble shooting a couple of issues.
I have had a couple parts break on me. Slide release on a Gen 2 17 and a slide lock lever. Now when I buy a used one I just strip it, replace everything except those good old machined extractors.
For most of my other brands I usually just keep springs and RSA.