there were 2.6million fiat 600s made in total, from 55 - 69.
i would assume the only way to find out how many were imported to the US is to ask (and pay) the NHTSA.
there is no way of knowing how many of the improted examples survive today, but i bet many of them have died over the years.
i would expect that finding a 600D will be very hard. the newest example is already 41 years old.