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I am currently doing some testing to see what the results of 1 ad unit vs. 3 on one of my websites.
The results so far (around 20 days) is that one ad unit with only two ads (the one called banner) - generate the same revenue as before when i had 3 ad units with a total of 8 ads showing.
The next test i am going to run is to replace this ad unit with one with 4 ads vertically (medium or large rectangle) to see if this creates different results.
A fair number of pages maybe, maybe 100 or so, have a single 250x250 adblock. The performance of these pages runs from "why bother" to "very good", I haven't got a clue why, someday I'll spend the time to figure it out.
25 or so pages have a 120x600 regular adblock with a 120x90 Adlinks unit above it. These are the worst performing pages on the site, mostly because of the way they are used. It doesn't really matter to me because the ads are in the only spot (righthand column) that I was willing to give up for ads, so they do what they do, and that's good enough. At this point it's there or nowhere on these pages as far Adsense goes.
My pages are ad-rich in keeping with my long-standing concept that content (or in newspaper parlance, copy) is only there to take up space in proximity to the ads.
I write solid content - have for more than 20 years - but I am a realist. It's worthless unless somebody pays for it, in this case, advertisers.
The more the merrier. I write some lengthy articles at times - 2000 words or more, and I now break them into 2-5 pages. Works for me.
My simple-minded formula: more pages + more traffic + more ads = more revenue.