That's a tough one to answer...
It would require VIN info from MartiWorks against every registered GT/CS in the country. But that's too impossible to do.
There are about 800 cars in the registry.
My "best guess" is that there are from 1500 to 2500 cars left. I think there are one to two cars not registered for every one that IS registered.
You can easily say that 1/2 of ALL Mustangs from 1964 to 1973 no longer exist, just by natural attrition, rust, accidents, etc...
Paul N.