I've been all over with a minimum of three as carry on and never had one questioned at a checkpoint anywhere in the world, but then I've never tried to take a 6 D cell cop club Maglight.

A small nickel size single LED on my key chain, and a penlight in my zippered ticket case. Uses AAAA batteries(not a typo) and is the size of a ball point pen, but very bright. Drop something under a seat with dark carpets and upholstry and its dark down there. The third one is where most would never think about. Another penlight in my little first aid kit. If you need it, the lights might be out. . .