Let's see... The light on my belt is primary for bagging the poop -- I can hold it in my teeth freeing both hands; the other 1xAA is back-up.
Then there's coyotes and at times at the coyotes discretion they visit the park in the dark. I like a light with reach to see them and now that I think about it, that's a critical function so it needs a back-up too. So I'd say four lights would be minimum and since I dislike accepting minimums, five.
Outside of the "Stories" bounds: IMO, 5 are needed because there is no single ideal flashlight for everyone and everything has been produced so far. But see my 3D printing thread for a possible solution I'm researching. However, I would disagree on a mandatory EDC flashlight backups. A good EDC flashlight should not require a backup. It must be virtually indestructible and warn the user about its power source depletion well in advance (or allow quick and easy battery replacement with an EDC or widely available spare - that's what I prefer; I plan to make a shell for my EDC 1xAA with the spare AA compartment integrated).