The rules say that you should only play 2-6 players, and that 5 is optimum for the Millenium scenario. I wonder what squeezing in 7 players would do.
Well the 6 player limit is presumably down to the counters. As to why the millenium scenario is a 5 optimum I guess some of the kingdoms are reckoned to be a little weak.
Looking at my historical atlases I imagine an extra kingdom or two could be added to most of the scenarios. Possibilities that spring to mind are; Castile, The Teutonic order, The Union of Kalmar, Hungary.
I would imagine the main problem with 7 or more players is increased downtime.