Seems like there's been a lot of nonsense talked about Rolls Royce cars here while I was at work and unable to set the record straight.
Rolls Royce only ever produced one V12 car engine, in the 1936 Phantom III. Most of the engines they used over the years were straight sixes, with the last real Rollers using a V8, in fact Rolls were the first company to use one in a production car (in 1905, a decade before Cadillac).
As has been pointed out, modern "Rolls Royce" cars use the BMW derived V12, and as a point of reference BMW advertised the earlier version of their V12 in the late 1980s by balancing the UK pound coin on the engine. At the time I was working in a Nissan franchise, and discovered I could do the same on a Nissan Micra 1 litre inline-4. Beware what you believe in the marketing...
EDIT: May have been a 50 pence piece.
To correct a misapprehension Specmaster, the W12 is not two V6's stuck together, the W-configuration uses a double stack of cylinders per bank. The Aston Martin V12 of the late 90s was in broad terms two Mondeo V6's joined end to end.