100 million users on a Prime yearly plan ($139) earns Amazon 14 billion a year.
100 million users on a Prime monthly plan ($14.99) earns Amazon 1.5 billion a month.
100 million users on a Prime Video monthly plan ($8.99) earns Amazon 899 million a month.
The show went for two months so any month subscriptions would be earned twice if it was just for the show. Of course the "real" earnings is complicated because it isn't all new sign ups. All but the Prime Video plan goes to other things as well. The streaming platforms all use their own internal metrics for when content "earns" enough. Since we know that Rings of Power broke records for sign ups it is clear that it likely will be seen as earning its keep.