Hello Armin.
The way I always read it was "You have 100 active users included. You can buy more". And active users are not online users, but users with isActive = True (it is up to you to see if they are disabled after weeks or months of not logging in).
Inactive users don't count. If you have a Standard license, you have 100 active users. You can change the contract to have 1000, 10000 or any other number.
The pricing used to be based on the complexity of the applications, but that was an incentive to avoid doing apps in OS. Now you pay based on the number of people that benefit from you having OS. You can develop your full business in OS without paying more for it than a similar-sized company that has a single app.