I just happened to see this page again after a long while. I see you wonder, whe there are only 7 Churfirsten, though there are additional summits. My explanation would be as follows:
The name Churfirst of course on one hand has its origin in the name of the town Chur. The other, quite obviously for us Germans, is the similarity to the title "Kurfürst". The Kurfürsten were the seven (!!) electors of the roman emperor, all powerful dukes and princes in their own right. While the German empire (Holy Roman Empire od German Nation) lasted, there were always seven heditary ones. The 30 years' war was started because the Emperor tried to exchange the protestant King of Bohemia with one of his Catholic dukes.
I think the original ones were: Archbishop of Mainz, Archbishop of Köln (Cologne), Archbishop of Trier, Duke of Brandenburg, Sachsen (Saxony), King of Böhmen (Bohemia) and Duke of Pfalz (Palatinate).
Great info! Thanks Gangolf.