Is it better to have around 3-4 ads on each page? I have a blog-like template, and I'm thinking about putting some ads after each entry, one on the sidebar, and on at the top. Would that be overkill? Sometimes, the Adsense doesn't even show up, I guess because of the numerous ads. I also heard that the higher paying advertisers do not show up in numerous ads, or the cheaper paying advertisers show up more. More ads seems like it more chances for people to click on ads, but if the click is worth less, I don't know if it's worth it.
the more ads you have, the harder google is trying to find them relevant meaning if you have less, those few ads will be higher paying than the rest. In some cases you're better off displaying less so they are worth more.
If there is a blog-like template with new entries, would it be recommended to place ads after each entry or perhaps only after the first new entry? And then one on the side and one at the top?
Take a look here for some more information about layout tips: http://labnol.blogspot.com/2005/07/google-adsense-layout-tips-for-maximum.html
That the max ad units you can have on a page is three. So if you put more than 3 ad units on a page, you are going to get a lot of white space in those areas beyond the 3 ad units.
Google displays the highest paying ads first. If you have multiple ad units (3 max), the last ads in the last unit will be the lowest paying ads. If the ads being displayed are highly relevant, then a single ad unit will provide the highest revenue. If the ads are not particularly relevant, multiple ad units will be best -- because it gives the visitor more choices to click on -- a little revenue is better than none.
The answer is, it depends -- every site is different. You should test combinations of ad placements and numbers of ad units to determine what works best on your particular site. You can use custom channels to track the results. For hints, check out: http://adsense.blogspot.com/2005/10/make-most-of-your-ad-units.html