Are the sources available for the approximate number of each manufactures cars on the road with level 2 features overnthe last few years? I don't understand why

Are the sources available for the approximate number of each manufactures cars on the road with level 2 features overnthe last few years? I don't understand why the NTSB would not at least normalize by the number of cars? Surely they have good estimates for those numbers no? Aren't all vehicle Vin numbers in a government database some where?