I would say it doesn’t really matter how many you promote, as long as you promote the ones that perform for you.. If you have a selection of 10 to choose from, then when one stops performing or takes a down turn, you have others to choose from to test in the same positions…
I think if you can really dig deep into details about why you are promoting the brands you are, it will give your players more confidence that you know what you’re talking about too… which should result in higher conversions for you 
IMO it would be better to give a detailed option for 8 casinos, than just a banner, link and half amount of text for 10.
Also, better not to put all your eggs in one basket 