One really bright one if it's daytime. Two at night, because one can fail. I don't use anything less than 30 lumens. What I have in the stable now is a Magicshine, a Serfas Shield, a Knog Blinder 4V and a Cygolite Hotshot.

I have some PBSFs around but they're too wimpy. I might throw them on my helmet as a 3rd, maybe, if I am in the mood.