Hi Marcus,
I think you’ll find a number of different answers to your questions here.
1. I think it’s best to find the right number of properties to promote for your site. This will take some experimenting and trial and error on your part. I know some webmasters that do well advertising a few properties at a time, as well as some whom do well promoting 20+ properties at a time. This is a variable factor and you will just have to try it out for yourself. That being said, to start off, I would recommend concentrating on a few properties and build from there.
2. There are many factors to consider when looking for an affiliate program. Not only payouts, but reputation, availability of affiliate managers, a willingness from the program to work towards your goals, statistical offerings, and conversion ratio of the properties.
Good luck in your quest!
Regards,