I mean, if only one AdSense ad unit, visitors may only clicking at high-value ads but low-value ads. Some will be relatively good results, although the rate may drop.
No dollars with no clicks so a couple of ads in different format seems to give me the best performance in the amount of money i earn.
the first ad unit displayed will normally give you the best-value clicks. if you will display one ad unit, it is best that you position it based on heat map. displaying 3 ad units, will decrease the chances of the 1st ad unit of being click. IMO, it still wouldn't hurt because you still earn, if the visitors click on the ads
There are two thoughts to this: One is that if you limit the number of ads to be clicked, the clicks will pay more, thus translating into higher revenue. The other is that if you offer more opportunities for the visitor to click (i.e. more ads), then you'll make more. You should try both options and see which one pays off more. It is not the same for every site.
I think the best thing to do would be to serve to blocks. Its a nice compromise between serving the best ads and enough to have lots of possibilty of getting clicks.
I have a leaderboard on one of my sites. I am thinking of replacing it with 5 (125x125) buttons and having them side by side. Would having 5 of these go against TOS for having too many?
Think about it from the end user perspective. When I see three ads on a page, I tend not to come back. Whereas if there is just one ad, even if it's prominent, I don't care so much. When people see a page full of ads, they worry about spyware. When people see a single well placed ad block from a respected publisher like Google or Yahoo, they read the ad. Have you ever seen 3 ad blocks on a page, read through 12 ads, and then clicked on one? It's unlikely that happens very often at all. I'm of the mindset that you should work hard to figure out the best placement and then be satisfied with your work. The ads-all-over-the-place philosophy is a throwback to when CPM ads were extremely popular and advertisers didn't have quality control demands. About.com has a lot of great info, but the fact that they try to set 12 foreign cookies any time I visit, coupled with a 60 word teaser surrounded by ads makes me just stay away from the site. That and the fact that you have to go through 4 of those pages to get any quantity of decent information. Another factor is one that you brought up already - fewer ad locations means the best revenue generating ads will be shown. Concentrate on user experience, and you will make more money in the long run.