When you start the game, your Company Level is 1.

As you level up your girls, you can sell your Company after reaching the required number of level ups. Each time you sell your Company, the Company Level increases by one. The requirement for selling Company increase as well.

Note: Required levels represent the cumulative level of all your hired girls.

The top banner that's displaying your current income (middle of the screen) is your Income display. It also shows you how much you have progressed before you have to sell your company.

It will flash green once you're eligible to do so.