How many copys of diablo 3 have been sold?

Answer 1

20million units as of 30th June 2014 across all platforms including the expansion and digital sales.

This info is from Activision Blizzard's 2nd quarter report.

Answer 2

Within 24 hours of its release more than 3.5 million copies of Diablo 3 had been sold. Released in 2012, it sold over 12 million copies that year, which was the highest of any game.