If you have a lot of ads on your site, do you make less money? I currently have 3 content and 1 link ad on my site.
it is not like that only your high bidding ads will be spread out so use a tracker to find which ad unit pays the highest and implement more.it is better to have some content instead of loosing a visitor for $0.01.
It depends on your niche. If there are a lot of advertisers, then it's OK to put 2 or 3 ad blocks. If it isn't, stick with one block as I do.
It also depends on how well you blend, place, etc. Sometimes too many ads make a site look bad, but if you design it well, four ads can blend in well.