There are two types of subscriptions: Per User license (monthly or yearly) and a Site licenses (yearly).
If you have a Per User license, you only need to purchase licenses for the users who need it. This can be adjusted monthly or per year, depending on your type of subscription. Read here how you can adjust the number of File 365 subscriptions in the AppExchange by yourself.
The Site license is an organization-wide, yearly license, for an unlimited number of users. As you can assign licenses to any user, you don’t need to purchase any additional licenses nor any other administrative handling is needed.