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.
My own experience has been that the best games have been between two or three people and that it can probably accommodate four or five. Has anyone experienced an interesting, enjoyable, challenging game with six to eight players? Or has it gone the other way with "too many cooks spoiling the broth?"
In the interest of keeping answers relatively objective, please answer this question based on experiences, that is games that you have actually played, or at least observed. If you want to cite a book or study discussing based on the author's experiences (either first hand or computer simulated) that would also be fine.