You don't HAVE to have any. If you're asking how many you're ALLOWED to have then I strongly suggest you read the TOS and Policies. (you agreed to them when you joined but apparently must not have read them too closely) I am not going to give the answer here but suggest you read the TOS and Policies again for the answer because there may be other things in there of interest to you...information that may keep you from having your account banned. https://www.google.com/adsense/support/bin/answer.py?answer=48182&topic=8423
I would suggest you to have one advertisement per website page, it don't make website overcrowed with advertisement and still look professional.
add or ads ? , i think can set unlimited ads zone in one page but please Read TOS and Policies first .
You can run fewer ads and still make decent money if you put the ads in the right spot. Use their heat map as a guide https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=17954
Generally, you want to keep in mind that the more ad units that you have, the higher chances you will end up with low paying ads making their way through to appear at your site. Unless your niche is something like finance and insurance, 1-2 ad units should be good enough.
The more ads you will place on your website the chances are that less revenue you will earn It also depends on the niche you are targeting. In short you can place 3 Ads unit, 3 links units and 2 search boxes but your target should be that the ads blend with the website site content in such a way that they don't scream that we are ads. You should focus more on ads placement rather than number of ads Tehmina