It applies to app builds and is identified by unique ids of the app (Android package name and iOS bundle id) so if your Amazon apk has the same package name as the one in Google Play this only counts for 1 app build. So in your case you would need 1 license for iOS and 1 license for Android (given the package name is identical).
about 10 years ago
Does the "per-app" pricing apply to each of the Android and iOS builds of an app, or does it apply to both app builds?
If, for example, we create both an Android and iOS app to cover central Paris and put them in the Apple, Google and Amazon stores then is that one SDK license, two SDK licenses, or three SDK licenses?