Finding out "how many registered Whatevers are left" is something you will probably never gain access to. After 9/11, US privacy laws were strengthened, and the Privacy Act of 1974 actually gained some teeth, as well as enforcement. It is impossible to call your state DMV (or equivalent) to gain that kind of information. Used to be, you could pay $X to get that. You could even get registration information on cars or trucks that were not in your name, also for a cost. No more. In fact, you can't get info on a car you USED to own anymore, through the State. You sold that '71 440+6 Challenger back in '92 and wonder where it's at? Yeah, right, sure.
I've read in several sources that in most circumstances, a particular year/make/model older than 15 model years, you can figure 10 - 15% as a good basis for total remaining, period; registered and/or stored away, but not including junked cars. With certain models, there are always exceptions. For example, all Hemicuda ragtops are accounted for. There are Viper guys that can tell you how many of what year are left, and be very close.