If you have multiple websites in which you want to use fullPage.js, there's nothing to worry about. That's not a problem at all. 

All fullPage.js licenses (except the OEM/SaaS license) allow you to use the code on an unlimited number of domains, servers, and projects, regardless of the environment type (production, QA, etc.)

When using the OEM/SaaS license you'll need a license per project that requires distribution rights.

Learn when you need the OEM/SaaS license.

Bear in mind that this works differently for fullPage.js extensions, in which you'll have to purchase one or another license depending on the number of domains you want to use it for.