if we need to initialise Admob SDK then should we initialise it for once at MainActivity.class or there's need to initialise it in every activity in which Admob Ads are present? 

As per this link : <https://firebase.google.com/docs/admob/android/quick-start> "To initialize the Google Mobile Ads SDK at app launch, call MobileAds.initialize() in the onCreate() method of the MainActivity class." 

So should we call it only once for every Ads in different activity?