A WebmasterWorld thread has a discussion around why Google might not allow you to add more then three AdSense ad units per page.
WebmasterWorld's wyweb, a senior member, has a great theory. He explains that, imagine your web site is your house. Would it be fair to your neighbors to paste billboards all over your house and lawn? Not necessarily.
Zoning laws still apply. You can try to have them changed or you can move to a neighborhood with more favorable (to you) laws.
Is Google providing some type of zoning law for ads on the Internet? I am not sure. I think it might have to do more about ad inventory. If you have a page about a big blue pineapple chair (I love going back to that post), then how many relevant ads can Google possibly show? Nine ads seem like a lot already and typically, Google will show 9 or 12 ads, at most, on a web search page, query dependent. So, I would think it is more about inventory control.
Overall, you can do very well with three ad units. Just test your placement and color schemes and see what works best for you.
Forum discussion at WebmasterWorld.