if you have only one ad per page, it will usually be the highest paying one and will probably result in higher ePC, but possibly a lower CTR.. If you have 3 ads per page you will probably get a higher CTR, but also the possibility of a lower ePC.. I have found that less usually = more overall earnings.. But even when it doesn't i usually still display only 1 because i think my visitors appreciate it more..
I tested it and the answer is yes. I have only 1 header ad now and the income is almost the same to my site with 3 ads placement (a little bit less). Even though I earned less but its a good thing since I don't like to put too much ads on the site anyway.
in my opinion, more ads units on one page means less adsense earning. it's better to use one ads unit and one link unit on one page
I've tried it and got less earnings with less ads but I think it all depends on the design of your site and where you place the ads.
hmm! i prefer less ad unites! i have placed only 1 ad unit on my site but on some areas i have got 2 but not more then that!