To keep Project Wonderful moving plenty of cash around, it's important to give advertisers plenty of choice - but to keep supply in proportion to demand. If you're earning less than fifty cents a day on your site and you've been going for more than a month, you may want to use fewer ad boxes.
Each and every advert on your site is competing with the others on the same website, and if you have too many, then there's no good reason for an advertiser to make that all-important first cash bid.
Remember that most advertisers are looking for the best deal for the least money. It's unfortunate, but true - nobody's going to just hand you a fistful of dollars for having a cool comic or blog, especially if there are a dozen button ads, a leaderboard and a skyscraper to choose from on the same site.
Competition is always a good thing, but you shouldn't compete with yourself!
It's hard to say what your ideal number of ads should be as it's going to be different for everybody, but as a general rule of thumb, if you ever see "Your Ad Here" images on your site and you're not earning any money, you could probably do with fewer ads.
It's good that advertisers have plenty of choice - but there's a big jump between giving them choice and flooding the market!
Project Wonderful Talk
http://www.projectwonderfultalk.com/article.php/preventing_market_saturation