I’d say a lot of it depends on your site, what the theme is behind it. If it’s driven by content, I’d say you shouldn’t limit the site to a fixed number of operators. If you’re talking simply about banners, pick those operators with the strongest brand, the strongest calls to action, the best retention policies and who offer you the best deal!
My suggestion would be to try out a fairly large selection of operators on your site, move them around and find out what works best on your site and makes you the most money! :hattip: