Blair has slept with Chuck, Nate, Carter, Jack, Marcus, and Louis. So altogether six people.
In Season 1, she Lost her virginity to Chuck, who could forget<3. Then she pretended to lose her virginity to Nate.
In Season 2, she started dating Marcus, and at first he refused to sleep with her. par 2x03, it was made known that they were sleeping together. After they broke up, she slept with Jack in order to get him to bring Chuck back after his father died. Once Blair's life started going up in flames because of her rejection from Yale, Blair began sleeping with Carter, but it only lasted a few days. After this, Blair and Nate got back together =(. They were definitely getting it on. But then when C&B reunited in 2x25, they definitely had some make-up sex.
Season 3 was all about Chair! Blair wasn't sleeping with anyone else. She didn't sleep with Jack this time, but it did lead to the Chair breakup =( She went on a few dates with that Cameron guy, but they definitely didn't have sex.
In Season 4, Blair didn't have sex with Louis when she was in Paris. When she got home, she and Chuck had lots of hate sex. Pretty amazing! Blair and Dan kissed, which was disgusting, but they never had sex. After, she began dating Louis again, and they had sex before they got engaged. Blair and Chuck had sex in the 4x22, when they (hopefully) conceived Baby Bass!