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.