You need 1 license per project you use Schedule-X premium in. If you for example have a dev agency with multiple customers and projects, you need 1 license per project.
A "project" here, however, is not defined by the number of users or the number of instances your software will be deployed to. Think of it more as a "product". If you have a SaaS platform with many customers that maybe even self-host your platform, that still just counts as 1 project. After all, for you it's just 1 product.
If you are uncertain about how many licenses you need, please reach out to tom(at)schedule-x.dev