If you include Javascript, it's very common.

Beyond that, I've pretty well stuck with one language in production work for about five years. But that's only because I've mostly done modest-scale web app development, which really only requires one backend language. If you stick to web apps, you can go a long way with a single backend language.

But my current startup is about 50% web app and 50% Linux app. So we're finding more and more need for C.