Boostn allows you to manage multiple projects from within the same account. This way, you can use a single dashboard to control the on-site messages of many different sites.
Just create a new project for each site that you plan to manage with your Boostn account. To do so, go to the Projects page and click on New project.
Next, you'll just have to choose a name for your new project and you're done.
Also, keep in mind that each project/site has its own init code. Read the guide on how to install Boostn for more information.