To be honest I just took the numbers from http://wiki.answers.com/Q/How_many_car_dealerships_are_in_Am... which claims to quote the National Automobile Dealers Association. More completely that page says "more than 17,000 new car and truck dealers with approximately 37,500 separate franchises" so even without the 'separate franchises' the number is still higher.
But yeah, I agree that the numbers are amazing...