There is no set maximum - just think about the user experience when deciding how many you show, as interstitial ads are more intrusive than banners.
Loading a new ad in onDismissScreen() is allowed, and is a good place to load the next one, to make sure it's ready to go by the time you want to show it. Of course, you should only call show() at a good stopping point, such as the end of the stage.
Cheers,
Eric