We love our pups!! Misu, RIP 28 Nov 2010. Tikka, RIP 11 Aug 2011.
On the board? One at a time if you're talking about the city spaces corresponding to the various types of nobles. No limit if you're talking about the history spaces (hexes that record if you've ever been in a city).
If you think of them as nobles rather than cubes the answers become more obvious.
There can only be 1 King of England, so only one noble can occupy each position until he is killed and replaced.
There can be any number of "dead nobles" in the church space with the cross, this is where they are all buried.
There can be any number of yours and other's cubes on the 7 tiles down the left side of the board. I refer to this area as the "history book" area, because it shows how many nobles of each rank each player had throughout the course of the game.