Strange as it may sound, I doubt everyone could agree on exactly what counts as a "Model A". One approach is you could go strictly by registrations, so if the owner calls it a Model A and the state agrees it's a Model A, it must be one. But, there are many cars registered as Model A's that no one would ever mistake for something that rolled off Henry's assembly line - and that's not limited to hot rods either. Also, that approach ignores the many actual Model A's that are not registered for whatever reason.
Here's some questions that need to be answered before you can begin to count them:
How many Model A parts do you need to make it a car, and on the other hand, exactly how many parts can you remove or modify from a Model A before it isn't one anymore? For example, does a Model A chassis with an engine count as a Model A or are they just parts?
Is there any single part or group of parts that identify a car as a Model A to the exclusion of any other car? Or does it simply need more than half Model A parts? What if it's more than 50% reproduction or fabricated parts instead? What if it has all Model A parts but they are cobbled together to create a car unlike any Model A that was ever built?
Let's say one criteria is that it has to be a drivable car. If you have a disassembled Model A in the middle of a restoration, is it still a Model A or just a pile of Model A parts? Most people would probably agree it should be counted as a Model A, but what if it's never reassembled?
Are all of these Model A's or aren't they? I don't think I could say for sure in all cases. It seems like no matter how you try to define what a Model A is, there are exceptions that break it. Which is why counting them is so difficult.