There seems to be a rule in tool engineering that the probability of failure increases as the square of the number of moving parts a tool has.
(and that is just how it is.)
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I have had some gear fail but to carry a spare of everything that might possibly fail would be too much.
The result is that I carry spares of what is likely to fail and that varies depending on how critical having the item is, and how easy replacements are to get.
If I am heading into the bush for a month I will likely have at least two belt knives with me plus a folder.(Plus saw and axe)

If I am going for a walk downtown one small pocket knife like the Swiss Army soldier is enough knife for me to carry.