Their official forum reports 20,000 members now. Since you can only read and sign in to that forum when you confirm your steam account during registering, this means, that AT LEAST 20,000 people have bought into the Early Access project alone. Not counted those - possibly even more? - customers who bought it and just do not care to visit forums. That is not rare. I for myself for example have never cared for the forums of some of my most popular games. Most racers do not care for online racing, Kunos own research showed that at maximum one in five races online only.
Conclusion of it all? I guess their sales numbers do way much better than I would have hoped in even my best dreams. Congratulations, Kunos! Well deserved.
iRacing, you better get a thick warm coat soon.
In 2011, I found numbers claiming they had 11-14 thousand customers then, and I believe I read some month ago that in 2013 it were about 20 thousand. I would not be surprised to learn that AC in its unfinished state already has overtaken iR in customer numbers.