A project is your future site. It may consist of several pages (a portfolio site, magazine or blog) as well as of just one page (a landing page or longread).
What makes it different is settings: a certain style (e.g. fonts, colors) is assigned to the
whole project, you also assign a domain. Thus, if you want to create several interactive stories (long-reads) in one style (fonts and colors) and assigned to the same domain, one project would be enough. If you are after several independent pages with different styles, which will be assigned to separate domains (e.g. landing pages for different customers), then you may need several projects.