Or try the state Motor Vehicles Department. Many states offer special license plates for antique or vintage cars, and they should be able to tell you how many of them have been issued.
That wouldn’t exactly tell you how many are driven regularly, but presumably they are driven at least occasionally (else why buy a license plate).
One of the biggest limiting factors on mileage is because almost all collector car insurance policies have an annual mileage limit (normally somewhere between 1000 and 5000 miles), and the special “antique” (or similar) registrations / plates also have an annual limit.
You can certainly register and insure a classic car as a regular street car, but then you’ll wind up paying more for insurance (classic car policies generally have really good rates) and if the car is a loss, regular insurance will go “That’s old; it isn’t worth much; here’s a check for $500”.
I have a collector car on an unlimited-mileage policy (rare) with a stated value (loss payout) of well over $100K, and it costs me about half as much as the 9-year-old BMW I insure as a daily driver. For people who have larger collections, the incremental cost to add another car to a collector policy can be as low as $100/year. I think the principle is “How many of these can he total at one time?”.
He probably meant that replica parts are still commonly available, not that the original manufacturer still offers spare parts. As an example, take a look at this selection of ads from Skinned Knuckles, a magazine for the car restoration community. AFAIK, Ford stopped producing the last replacement part (the complete engine) before WWII.
Excellent points, and this really gets to the core of the OP. Even if there are still 500,000 Model Ts “on the road” (and forgive me if I’m a little skeptical about that number), for all of these reasons, few, if any of them, are going to be their owners’ “daily driver” car. If they’re still running, they’re likely to be owned by collectors, museums, etc.; they might be taken out on the road a few times a year, or brought to car shows, but I don’t think that really qualifies as “driven regularly”.
If you put the time, money, and effort into restoring an antique car (or keeping an antique car in good condition), it’s highly unlikely that you’re willing to subject it to the risks inherent in driving it every day. If you have a car which you drive every day, even if you take extraordinarily good care of it, it would be quite exceptional* to keep it going for decades. At 10,000 miles a year (not an outrageous number for a “daily driver”), you’re talking about 700,000 miles on a 1941 car now.
For what it’s worth, I saw a Model T being driven earlier today.
Oh, yeah, this is the time of year (around here, at least) when you see that kind of thing. A beautiful summer afternoon, take the antique car out for a spin. 
Heck, that can be true even of regular cars. There are lots of people here in Minnesota who have a car that they only drive in the summer, and then put it into storage over the winter, to avoid the dangers of winter driving, and especially the salt used on our roads in winter.
BTW, How Many Left? tracks the number of UK registrations for a given model over time. That’s because certain UK registration data is a public record. BTW, for non-UK folks, a SORN (or Statutory Off Road Notification) is more-or-less the equivalent of “up on blocks in the driveway with no plates” in the US.
No, not on early MG’s. They weren’t imported officially.
All my others, except for my 1948 MG-TC, have some type of bumper, and they’re all chrome as a proper bumper should be!