Take Two has revealed that Grand Theft Auto IV has sold nearly 20 million copies worldwide. The franchise as a whole has sold over 100 million units worldwide.
They also note that since last year, Red Dead Redemption has sold over 8 million copies worldwide and garnered over 150 Game of the Year awards. Additionally Civilization is 2K s most popular franchise, with over 10 million in sales worldwide.
Source: Take-Two Games