The rules say that the game is for 2 to 8 players. But there are only eight monopolies on the board, meaning that in a game of eight players, there would only be an average of one monopoly for each player. Meaning that some players probably would have no monopolies. And it might be hard for any player to get a monopoly (except, possibly, by trading). Then you would have a great divide between the propertied, and non-propertied players.
But six players is mandated in some versions of Tournament Monopoly. Are there any tournaments that feature more than six players? Do other tournaments have five, four or three players more frequently than "six"
In the interest of keeping answers relatively objective, the best answers would address tournament experiences and results, or cite a book or study using an expert author's experiences (either first hand tournament or computer simulated).