The Philosophy Documentation Center‘s mailing lists provides a breakdown of how many U.S. and Canadian philosophers claim various areas of specialization. I don’t know how accurate the list is—it gives a total count of approximately 14,600 philosophers, including about 2,500 graduate students and 450 independent scholars—but I don’t have any reason to think it is wildly inaccurate. One downside is that some of the areas are drawn rather broadly (e.g., “history of philosophy”, “contemporary philosophy”).
The most popular areas are ethics, history of philosophy, analytic philosophy, social and political philosophy, and metaphysics. Here’s the whole list:
You can view this table, along with prices for the various mailing lists, here. I’ll just note that given the price floor they’ve set, along with the bulk discounts, the most expensive kind of philosopher to acquire information about is someone who does philosophy of history, at $1.15. Meanwhile, ethicists are a steal at a little over 9 cents a head.
(via Sally Haslanger)
UPDATE (10/9/15): Elizabeth Stombock at PDC provides some information about how this data is collected:
These lists are drawn on the information in the North American and International Directories of Philosophy and Philosophers. We obtain the data for these by soliciting information directly from the philosophers themselves or their affiliated universities, and supplement it by gathering information from university web pages and the philosophers’ personal websites and CVs. Each philosopher in our Directories is permitted to select up to six specialties from a predetermined list, which our system then sorts into the categories you see on that mailing list chart. Anyone can view and alter their entries online by creating a free editorial account on our website.