I try to keep it varied, and stick to about 1 or 2 per
network. This is because one will be rated higher than another, and if visitors go through the reviews and notice that two are almost identical, but one is rated better, they are rarely going to sign up for the worse room and so there is no real point in having it there. An exception on my reviews page is that I have PokerRoom as well as Hollywood Poker, which are from the OnGame
network. PokerRoom has better ratings, but I promote Hollywood because of the large bonus that it offers, which will attract the new players.
Although If there were two or more big and popular rooms on one
network I wouldnt have a problem with promoting each of them on my site.