I usually use two taillights especially when a light is highly directional. It also depends on where I'm riding. In traffic with a lot of cars, at least 2 taillights plus at least 1 sidelight (or spoke lights ). On short local trips or when it's not very dark (dusk or overcast), I use less.

Often I also carry back-up lights in my bag in addition to those in use, or spare batteries, or both.