The numbers represent the current level of the guy. To level up a guy, you will need to film them using the Film studios.

There are 6 studios where you can film your guys: 3 that are unlocked with cash and 3 studios that are unlocked with diamonds.

The level of each guy is determined by the sum of the levels of all posses the guy can film. There are a maximum of 14 poses a guy can have with only a part of them having all the posses available.

By leveling up your guys, you unlock additional sex position for them in the story and also increase your stamina for the BJ Challenge mode (as your stamina is the sum of all the guys' levels).