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 as a commenter pointed out, six players is mandated in some versions of Tournament Monopoly.
My own experience has been that the best games are between two or three people and that it can probably accommodate four or five. Are there accounts of interesting, enjoyable, challenging games with six to eight players? Or have most tallies gone the other way with "too many cooks spoiling the broth?"
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 or computer simulated). If you want to answer this question based on personal experiences, please limit it to games that you have actually played, or at least observed, and demonstrate some level of expertise.