Just ramblin', I'm a cheby guy:
The roadrace version of the Mustang about year 2000?, had 2 seats, but only 300 were made IIRC. These were street legal. Cobra-R?
There are new Mustangs for sale today that have 2 seats, but don't have VIN, just P/N, not for use on public highways. No idea how many. FR500?
But all Mustangs are built on a 4 seat chassis AFAIK. It's like saying the Camaro is 2 seater because they make some race versions without back seats. Yup, there are 3 P/N's you can order for full race 2012 Camaro. I think it has 1 seat?