You may display ads on more then one site using your google adsense account.  The more sites you have displaying revelant google adsense ads, the more profit you can earn.  You don’t even have to notify google of the new sites displaying ads. However  any new site that  you add must conform to google adsense program policies, because if your account gets terminated for any reason, you would not be able to display the Google Adsense ads on any of them.

Google gives you options for adding new ad-zones to your google adsense account, these ad-zones help you keep track of how well your ads are performing on that site or section of that site. Good placements of the ads are key thing to keep in mind, you dont want your ads to appear as ads, you want them to appear as a solution to your visitors.

How much of a profit can be earned having more then one site displaying google ads? 

How much you will be earning will depend on how much the advertisers are willing to pay per click. It will also depend on the keywords required. If the keywords are in high demand by the advertiser, you could receive more cost per click. On the other hand, low demand keywords will earn you just a few cents per click.

The fact is google adsense will tell you everything you need to know – for free. They have a support site that covers virtually everything you would need to know about the program and experience will teach you the rest, on the other hand thats where articles and guides pay off, keep reading and learn about the system.