All ads are linked to one account. But I have about 4. And they are all global on different parts of the site. Footer, Heading, Side panel, then also smaller ads for small locations.
I say that you don't need more then 5 max ads anyway
I run a handful of websites and blogs. The number of ad units on these blogs and website differ. ON some website and blogs I am not using any ads on the home page, I use three adsense units inside the posts. On some sites, I am using 5 ad units, one on the header, one on the sidebar and three in the feed or posts.