Frequency capping (per user)
Limits the number of times ads are shown to the same person.
But there are no visible changes in the app after adding capping frequency for an AdMob Ad Unit
Please note the following:
A change to frequency capping, such as changing the cap from two to three, can take up to a day to take effect.
A slight server delay can occasionally result in the frequency cap that you've set being exceeded.
From the AdMob Help Centre:
https://support.google.com/admob/answer/6244508?ctx=tltp&hl=en