My own wild guess is that about 500 people do the complete TransAm each year, about half from each coast. But I admit that this is just a wild guess based on very little information. Maybe 5 people start from each coast on each of 50 days. That estimate might be low--could easily be double that. You could also look at how many people get their pictures taken at the ACA headquarters each year, and then guess as to how many of those were on each route, and how many people skipped the HQ visit.
Anybody else got a guess and a rationale for how they come up with that guess?