According to this article on VentureBeat, Civilization VI has already shipped 1 million copies since the game’s release on October 21, making it the fastest selling title in the history of the series. While this doesn’t represent the actual number sold to consumers, it’s still a metric that outpaces all previous entries.
The article also said that Civilization V sold over 8 million copies, and that Civilization series’s total sales before Civ6 were 35 million units since the original Civilization came out in 1991.