I was searching for the infor and found it in the goolge website . They tell that. 


you may place up to three AdSense for content adsense units on one webpage. You may also place a maximum of three link adsense units and two search boxes on each webpage.

If you place more than one ad unit on a page, our system will display unique ads to each ad unit. Because the system automatically displays an optimal number of highly targeted Google ads on each page, in some cases, the maximum number of ads may not always appear. In this case, the ad unit will show as a transparent box, or will contain any alternate ad or color specified in the ad code.