Recently ,I received a message from Google. Google suggest me Adding any of our top performing units (336x280, 300x250, 728x90 or 160x600) to prominent sections of your pages will substantially increase your ad impressions and overall revenue. What do you think of the suggestion?
you can always try it and see. Go with three then try two then one and see what works and what doesnt. Trial and error every site different.
Careful how many ad units you add, to many and your site looks more like an ad, then useful content, then they might turn around and panda slap you. I still to 1 or 2, thats it. I also make sure it have the add fit nicely into the content and not stick out like a sore thumb.