Rewarded Interstitial Format specific policies
In addition to the common rewarded inventory policies, Rewarded Interstitial ads must also adhere to the following format specific policies ( reference https://support.google.com/admob/answer/7313578
):
Follow the Interstitial ad guidance for both Disallowed and Recommended interstitial implementation
Disallowed Interstitial implementation link - https://support.google.com/admob/answer/6201362
So there is no direct cap on number of times rewarded video should be displayed but You should place no more than one interstitial ad after every two user actions within your app. Please note that this requirement also applies when a user clicks the Back button within the app.