My direct ad sales have really picked up, so that means AdSense is probably going down to one per page, even on long pages. If I get a get a better EPC, all to the better (although it's pretty good right now)
However, more adverts do = more opportunities for one of them to catch the attention of (and be of interest to) that particular visitor.
I don't consider all ads to be equal, nor absolute numbers of ads to be the most important factor. If it takes 2-3 ad units to create a sufficient variety that one of the ads might appeal to each visitor, then that's what it takes.
I can't imagine that any visitor is looking for the opportunity to click any random ad that happens to be in front of them.
The relevance of the ad (to the visitor, not to the page) and its quality (some ads are junk) seem like they'd be more important factors than how many there are.
[edited by: SteveWh at 10:35 pm (utc) on May 13, 2012]