Frequency cap shows how many times the ad will be displayed for a visitor during the certain period.
Default Popunder frequency cap is 3 Popunder ads per 3 hours. Our Support Team can help you to set the desired frequency so you just have to inform us with exact cap which is required and specify the ad channel ID of popunder (which could be found in the Sites section of your account).
Please notice that decreasing Popunder frequency may decrease your revenue.