For licencing purposes an umbraco project is the collection of Umbraco instances that make up the deployment of a specific site or suite of sites

So for example you might have multiple developers, pushing changes to a development site, which then goes to a stage or QA Site, once things are approved they go to a live site, and that site may well be load balanced.

All the Umbraco sites in this process would be considered part of the same project.

If you have multiple clients all with their own live sites, then that would be multiple projects, and you would need a licence for each one.