To answer the OP, I would say not that often, but often enough for it to be nothing particularly extraordinary. Traffic on runway (i.e. a/c not vacating quickly enough or taking off quickly enough) is pretty common.
I've experienced three go arounds. One was because of traffic on runway (on approach to LGW), one was a faulty indicator or warning light in the cockpit on approach and the crew wanted to go around to check to make sure everything was in order (we ended up making an uneventful landing- albeit it was foggy, which might have had something to do with it) and the best one was a touch and go at LCY, with the explanation being a changing wind direction- there's obviously not much room for error at LCY!