One is none ... dropped and broke his *only* flashlight and sp ent two days in total darkness unable to find his way out ...
It's been awhile since I have done much hiking and back-country camping. But if armed with today's flashlight tech I'd certainly want to bring at least a couple of lights along with spare batteries.
For me, it'd be: two or three Malkoff M61 LED lights, and a dozen or so fresh 18650 cells; one on me (with some battery spares) as well as one set in the pack. The "split" load would help guard against, say, loss of the pack, or my carried unit getting damaged along with the pack's unit. Fully-potted LED drop-ins, and multiple spare batteries. That plus a decent nearly-unbreakable mirror ... that's about as good as I could make it, for visual lighting/warning.
Of course, these days, with emergency signaling, GPS and other electronica, it's getting easier to have the gear with you that'll avoid such scrapes to begin with. Better safe than sorry, when out in the back-country, on the ocean, down the river gorge, etc.
Back in my teens and twenties, I did uncounted journeys into the wilds, was in and around water, in spots where there was no communication and days' worth of travel to get to help. Can't imagine surviving it, these days, if faced with the same challenges. But I learned quite a bit. Including the "
One is none" adage. Very useful, that.
A stuff-sack emergency bivy sack (or two); an emergency "space" (heat) blanket; fire-starter; water purification and emergency water purifying "straw"; compass and spare; flashlights, spare cells; knives; several bundles of rope/paracord; etc etc.