If your question made sense, I would answer. Are you asking how many ad units are allowed on a page? Or are you asking how many ad units do you think it is good to display on a page? Or something else?
Here is my opinion: If you have high CPC, use just one adsense block 336x280 somewhere at top, middle. Google serves the highest paying ads in first block, and less paying on second and so on. If you have low CPC, like 0.03$, use as many ad blocks as possible.
3 content, 3 link, 2 referral and 2 search boxes, if i remember correctly. However, i'd not recommend the max. allowed number of units, you should decide it according to your pages, too much ads makes it look like a MFA.