http://boardgamegeek.com/boardgameexpansion/148931/coup-reformation is the expansion for more than 6 players.
It contains 15 cards; 3 of each rather than 2 of each. I think the rules from the game say how many cards to use for how many players, since someone quotes it as 25(5 of each) cards for 9-10 players. Since you use 15(3 of each) cards for 6 players, I'm guessing you add 1 of each card for 7-8, 2 for 9-10 and 3 for 11-12.
The rules for reformation are slightly different, as it has 2 factions where you can take an action and 1 money to the almshouse to change your faction; 2 money to change someone else's faction; or take all the money from the almshouse if you do NOT have a Duke.
You are only allowed to attack people of the opposite faction to you, unless everyone is the same faction where you can attack whoever you want.