If I had to get by with only one light, I think I could do it relatively comfortably with my Zebralight H52W. I use that headlamp daily, and would feel pretty comfortable depending upon it alone if I had to.
*BUT*, it is nice to have a spare light 1) on the bedside table, 2) in my car, 3) in my wife's car, 4) in my laptop bag, 5) in my weekend EDC/messenger bag, 6) in the basement by the circuit breaker box, 7) in the garage tool box, 8) in the junk drawer, 9) in my travel bag, 10) on my key ring, etc., etc.
Factoring in having backups of a few lights I really like on hand, I *should* be able to whittle it down to 15 or so and be content. I won't say just how far over qty. 15 I currently am, but that's only because I have no idea without actually rounding them all up. I suspect I'm very far over... (someone help me, please)