Google AdSense publishers can place up to:
a. A Maximum of three AdSense for content units (text, image or rich media) on one webpage.
b. A maximum of three AdSense link units (horizontal or vertical)
c. A maximum of two Google Search Boxes on one web page (it can be Google custom search as well but make sure the Google logo is visible).
d. One Video AdSense unit (this has been discontinued)
Google AdSense program does not permit AdSense ad code to be placed in email messages or newsletters. Nor are you allowed to add AdSense ads in software application, browser tools, extensions, etc.
The Google AdSense code may be implemented only on web-based pages.