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Ah, when you said ad units, I assumed you mean AdSense. In terms of non G ads, I run 4 per page with no issues. You could probably stretch this to 6 though.

So did you meet any other mozzers?

Im amazed that nobody commented on this. You may post up to 3 AdUnits per page. As long as you follow the rest of the guidlines, you should have no problems. I have a couple of adsense sites that do just fine as long as I follow the rules

"You may place up to three link units on a page, in addition to the three standard ad units, and two search boxes. Just make sure the page complies with our program policies. "

Hope this helps,

Are the pages mostly duplicate content? Do you know how many have been indexed?

If it's a lot, then yes, noindexing them will make it look like your site has dropped a ton of content. But if it's duplicate then I'd go for it anyway as it will probably help things.

Alternatively, how about removing them from the sitemap instead? They may still get found but at least you're giving them a clue that those pages don't matter to you.

Chicken and egg. Writing great content is a good idea...but if nobody can find it, nobody will link to it?

I'd expand on the above idea and suggest writing a great article then perhaps push it out through Digg so that people will see it and likely talk about/links to it. Works for me anyway.

Good luck!