500
it never met in plenary session, and most of the discussions occurred in informal, face-to-face sessions among the Great Powers of Austria, Britain, France, Russia, and sometimes Prussia, with limited or no participation by other delegates. On the other hand, the congress was the first occasion in history where, on a continental scale, national representatives came together to formulate treaties instead of relying mostly on messages among the several capitals.
In a technical sense, the "Congress of Vienna" was not properly a congress...