This is something I've wondered about for a while. Obviously, now that there have been six King Georges of the United Kingdom/the Commonwealth, we refer to the first one as George I. But would his subjects at the time every styled him King George I, or would he simply have been King George? We seem to often refer to the current Queen as Queen Elizabeth II, but I was wandering Wikipedia today and noticed that Queen Anne of Great Britain isn't styled as Anne I there, which sparked my asking the question here!
And an additional question: if the answer is yes, they were referred to as [Name] I, would this also have happened for the likes of Mary and Elizabeth Tudor, given that female monarchs were seen as absolute last resorts in that time period (and therefore they may not have anticipated there ever being another Queen Mary/Queen Elizabeth)?
Quite simply, when there has been only one monarch by that name, there is no need to differentiate them by number. If there was another British Queen named Anne, we'd start referring to the Anne who ruled in the 18th century as Anne I and the "new" Queen Anne as Anne II. This is why the current Pope is always referred to as "Pope Francis" rather than "Francis I."