I suggest that you test it to be sure. Sometimes it is better to have fewer ads in a cleaner page design. An ad needs to contrast to be seen and eventually clicked. More click = more revenues, however I am not sure that each click will pay more if you have fewer ad units on your pages.
Yep its true right, if the site already is on a too low paying niche, then more ads = more clicks and the per click amount is low anyway, so adding more ads is a good idea here.