If there are more than 10 players, we can't get a trader in everyone's hand. This may actually be impossible.
We could have a few more players and lose a Procession or two, right? Or do you really need all of them?
Well if we have 12 players then that's 4 processions lost, and that's really close to too many. I guess with enough other stuff gained it can work out. But 12 is not enough players I think, because we have to take the time to give everyone a trader and draw it first.
Oh, just realized, we can put all 20 events in the kingdom. Surely now...
You can actually have 11 players and still get everyone
else a Trader - Player 1 buys Messenger -> Trader, everyone but Player 11 gets one. Something like:
P1: Messenger -> Trader (empty)
P2: Stonemason/Lost City/Lost City
P3: Stonemason/Lost City/Lost City
P4: Stonemason/Lost City/Lost City
P5: Messenger -> Watchtower (everyone else gain Silver with Trader)
P6: Messenger -> Procession (everyone else gain Silver with Trader)
P7: Stonemason/Lost City/Lost City (draw Watchtower and Procession)
P8: Messenger -> Fortress (everyone else gain Silver, trash with Watchtower to put in hand)
P9: Messenger -> Procession (everyone else gain Silver)
P10: Messenger -> Procession (everyone else reveal Trader to gain nothing)
P11: Procession->Fortress to draw the two Processions and gain Catacombs. Etc.
Kingdom Cards Required: Messenger, Trader, Stonemason, Lost City, Watchtower, Procession, Fortress, Catacombs, Baker. That only leaves one slots (two if we want Young Witch to pass out curses?), I don't know if that's enough (we can get Black Market, but then we don't have 10 King's Courts). We're down 3 Processions, a Fortress, and a Watchtower (or whatever combination of cards we want to start with).