You can have up to three ad units, two search boxes, three link boxes and three referrals.
However with all that you may not have too much space left for content.
From Google:
Ad Placement
AdSense offers a number of ad formats and advertising products. Publishers are encouraged to experiment with a variety of placements, provided the following policies are respected:
* Up to three ad units may be displayed on each page.
* A maximum of two Google AdSense for search boxes may be placed on a page.
* Up to three link units may also be placed on each page.
* Up to three referral units may be displayed on a page, in addition to the ad units, search boxes, and link units specified above.
* AdSense for search results pages may show only a single ad link unit in addition to the ads Google serves with the search results. No other ads may be displayed on your search results page.
* No Google ad or Google search box may be displayed in a pop-up, pop-under, or in an email.
* Elements on a page must not obscure any portion of the ads.
* No Google ad may be placed on any non-content-based pages.
* No Google ad may be placed on pages published specifically for the purpose of showing ads, whether or not the page content is relevant.
