Depends on the model and year. A 95-98 A6 has a 2.8 liter. Later 2000 models have a 3.0 to 3.2 liter. Higher ends can have 4 liters. Usually displayed in stencile on the back of truck.
The 2008 Audi A6 has 24 valves.
The 2009 Audi A6 has 24 valves.
The 2010 Audi A6 has 24 valves.
The 2003 Audi A6 has 30 valves.
The 2011 Audi A6 has 24 valves.
The 2012 Audi A6 has 16 valves.
The 2002 Audi A6 has 30 valves.
The 2001 Audi A6 has 30 valves.
The 2006 Audi A6 has 24 valves.
The 2004 Audi A6 has 30 valves.
The 2005 Audi A6 has 24 valves.
The 2013 Audi A6 has 16 valves.