Don and Doug have good inventory plans. I like to look at having a spare for anything that will be a significant inconvenience if it failed and didn't have a spare, like a water pump.
I also could argue to have a lot of small hardware items, screws, clamps, eye bolts, loops, rivets, electrical stuff including wire, solder, glue, epoxy, etc. I use the little 8x12 boxes with ~30 compartments for storing them and have several of them.
For things that might be a minor inconvenince, especially bulky or really expensive stuff when you can get it reasonably quick I'll pass on.
Another thought.....
If I buy a spare pump, alternator, starter, etc and don't know the history of the current one, I'll replace with the new and use the old one as a spare, knowing it works and if I use one I'll buy another new one, etc., etc.