Depends on your platform. If you want to simply promote well respected high conversion programs I would select a number around 10, depending also on USA/non usa considerations. Then there is the model I try to promote where I list all online casinos and allow my users and myself to post active reviews good and bad and allow users to decide. I do see a lot of variations in this model and would frankly remove quite a few, but to note them for what they are is what I do.
Then there are other verticals, as many as you can do while sill enjoying it, and not working more than a CEO of a large corpration.