The ads that appear first pay the most. You don't want to have too many ads 'cause if people click on later ads you get paid little. I've had ads paying 1c.
Putting more and more ads won't make you any good for you. To earn more try to focus on bringing more traffic in and placement of the ads in a proper manner that will attract more people to click on it. Otherwise flooding your site with ads will not make any good.
Definitely 3 are enough, dnt have a good look with more ads. just get good colors for ads, title color, background color related to page design
Don't overload your site with too many ads. Instead think of filling it with good contents and limit the number of ads to three or to a maximum of 4. Remember : Visitors come to your site in search of contents and not to look at ads. More ads will only drive them away. Have more contents and traffic and enjoy the adsense earnings