Back of the envelop calculation. A PubMed search for "patch clamp" over the past 5 years reveals about 10k hits. If the average lab produces 1 paper per year, there may be about 2k labs that patch. I'm guessing that the average lab might have 3 people who can patch, so that gives us 6k active patchers. Hmmm, a pretty exclusive club!
You may check @ http://ionchannels.org/labs.php and see the active labs to have an overview of people using patch-clamp in their publications. It will just reflect how many labs are using this method, not the number of patch-clampers though