The Adsense rules stipulate that you are allowed to include 3 regular ad units and 1 link unit per page. The number you should use per page will completely depend on the structure and design of your website.

What you don't want is a webpage that appears like a collection of Adsense Ads. Such a site is valueless and will struggle to get any traffic unless you enter the world of spamming or decide to pay for traffic - both offer only diminishing returns. For a visitor to click on your ad means that they must want to go forward from your website to another website. If your page repulses them straight away they will hit the back button (read about Breaking the Back Habit in The Perfect Page) and return to the search engine that they came from to find another site that will answer their query more effectively.

Entice your visitors into your site and then cleverly place ads at points where they will be focusing as discussed in Ad Placements.

So how many is not too many? This website can easily absorb two ad units and an adlink as it does. It could also include another ad unit at the bottom of the articles which I may introduce over time.

As a general rule I'd start with more ad units than less, set up channels so you can monitor how each performs and then make changes accordingly. If you're building your website from scratch try and build it with enough flexibility to allow you to test different variations.

One Unit - A large rectangle (or at least the 250 x 250 square) inside the body of the page with the article text wrapped around it.

Two Units - Either

1. A more aggressive tactic is to include two large rectangles together either side by side or on top of each other. If you're getting a fair amount of traffic this will really improve your ctr and is worth tyring. Just keep an eye on your web stats to check it doesn't undermine your visitors experience which will be highlighted to you through a decreasing traffic rate.

2. The large rectangle inside the content and a skyscraper (preferrably the 160 x 600) in your left navigation. The right navigation converts much lower.

3. A large rectangle towards the top of your content and another at the bottom. This gives your traffic two ways out from inside the same page.

Three Units - If you can fit three in you can now mix and match, for example:

1. Two rectangles inside the content and one skyscraper in the navigation.

2. One rectangle inside the content and if you have a right and left navigation, one skyscraper in each of these (I know of websites this has worked well on)

3. One rectangle inside the content, one skyscraper in your navigation and one leaderboard at the header of your page.

4. Three rectangles inside the content - this can work very well if you have a lot going on inside the page and it is long enough to absorb the three ads allowing them to be placed far enough away from each other.

Including an Adlink Unit

Your one prescripted Adlink can virtually always be included as they can slot in to the smallest of places. Many publishers still neglect Adlinks as they have wrongly convinced themselves that the requirment to click on a keyword phrase before they can click on an ad is requiring too much from a visitor. To learn more about how best to use Adlinks go here.