Number of ads the GADAdLoader should attempt to return for the request. By default, numberOfAds
is one. Requests are invalid and will fail if numberOfAds is less than one. If numberOfAds
exceeds the maximum limit (5), only the maximum number of ads are requested.
The ad loader makes at least one and up to numberOfAds calls to the “ad received” and
-didFailToReceiveAdWithError: methods found in GADAdLoaderDelegate and its extensions, followed
by a single call to -adLoaderDidFinishLoading: once loading is finished.