I quite understand, my keyboard can't spell either at times. It's hard to say how many are left, because not everyone joins a club or mingles with fellow owners. The car clubs are not as strong here as they are in the UK, where most, if not all, MGB owners are now members of one of the several MG clubs. I would venture to suggest that most in the United States are not.
Some people attach excess value to a car just because it is old, and/or of low mileage, and just keep the car in the (often damp and cold) garage rather than driving it. I know of one rubber-bumper MGB less than a block from my home that is a low-mileage one-owner car but I have only ever seen it driven once, and then it had holes at the dog-leg panel at the rear of the sill, and flat paint. The same person had an early pull-handle MGB that she covered with a tarp in her front yard and allowed to just rot. I had offered tobuy the car for restoration, but she stated that she had promised it to someone. Eventually, the only person who got the car was the guy from the scrapyard.
Anyway, I digress. I see many MGBs around that do not attend car shows or club events, and several MGB owning club members are rather put off by the cliques and, dare I say it of MG owners, snobishness, exhibited by some owners of pre-MGB cars, and do not renew their memberships.
That being the case, we may never have an exact figure, but I would suggest that it is fairly high, with some more 'barn' cars yet to be uncovered.