I keep in inventory at least one full set of every screw and moving/loaded action part that's unique to a given gun design that I own. I can thus completely replace all the moving/loaded parts and screws on at least one example of every gun I own. There are some parts for which I have multiple stocks simple because it made economic sense due to their low cost - the shipping was essentially free.
Moving/loaded parts are those that either move against another part, like a trigger or hand, or are subjected to a cyclic load, such as a spring. One part I have several examples of is the Heinie Colt SAA wire trigger/bolt spring, since the OEM leaf springs tend to fail frequently and the Heinie wire spring is a much better design.