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.
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• 2013
  • 1,809 -  Model Year 2013 Audi S6 4.0T QUATTRO S TRONIC models sold in the US.
    •    115 - with Brilliant Black exteriors sold in the US.
    •    109 -  with Brilliant Black exteriors and Black interiors sold in the US.
    
    
• 2014
  • 1,309 - Model Year 2014 Audi S6 4.0T QUATTRO S TRONIC models sold in the U.S.
    •    454 - with Phantom Black Pearl Effect exteriors sold in the U.S.
    •    426 - with Phantom Black Pearl Effect exteriors and Black interiors sold in the U.S. 
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• 2015
  • 374 - Model Year 2015 Audi S6 4.0T QUATTRO S TRONIC models sold in the U.S.
    •    9 - with Havanna Black metallic exteriors sold in the U.S.
    •    7- with Havanna Black metallic exteriors and Black interiors sold in the U.S. 
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• 2016 (to date, 9/27/2015)
  • 546 - Model Year 2016 Audi S6 4.0T QUATTRO S TRONIC models sold in the U.S.
    •    140 - with Mythos Black metallic exteriors sold in the U.S.
    •    123 - with Mythos Black metallic exteriors and Black interiors sold in the U.S. 

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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).