I don't know of any way that you could even think to calculate what's remaining. I am sure that it is technically plausible to calculate how many cars are currently registered with current tags as all of those files in North America would be electronic. But getting all those agencies to talk to one another would never happen. Thinking of how many cars would be registered to owners but currently parked, stolen, cut up for track cars, derby cars, cars scrapped for parts would be mind boggling. For example I have the ownership for 2x vehicles that I cut up and took in for scrap metal as pieces years ago that are still technically registered to me but have been soup cans many times over by now.