Can i add this into many different websites using the same one account

Yes, a single AdPlugg account can serve ads into any number of websites.

AdPlugg will automatically serve the same ads to all of the sites.  If you want to serve different ads to the different sites, you can do this with zone or page targeting.  We plan to add a site targeting feature soon but much the same thing can be achieved by putting different zones on the different sites.  Take a look at the recipes in our Cookbook for help with zones and targeting.