Audi of America will look up sales numbers for you given you give them a VIN. I provided 3 VIN's to get the current 2013-2016 S6 numbers from them.
Currently, we have reporting in the above for the below. It would be great if others helped to make this more complete. Two options for you to help if you're so motivated: send Audi a mail to request the info yourself (see how to here), or drop me a line via the Contact page and give me the model year, exterior color, and VIN for your car (if you don't see an X for your color/year below).