It still sold pretty well on a single platform i.e Xbox 360. I reckon it sold more than half million. That too after the fact that the original Witcher wasn't even released on consoles. If they had targeted multiple platforms i.e PS3/360, the final sales number might have been close to 1 million.Witcher 2 sold more on PC than Xbox360? I didn know.
You just need to take a look at Crysis 2, a PC exclusive(In start), which sold about 75% of its copies on consoles. The number might be higher or lower, i can't remember exactly but Chandoo had posted it in the Crysis 3 thread.