Summary

The user count for your Confluence instance is calculated based on the number of users with permission to log in, for example: users with the 'Can Use' global permission. 

Let's say you have a license for 500 users and you have 500 active users who have Can Use permission on Confluence, everything will work fine as expected. However, your Confluence application will turn into Read-Only Mode right at the moment when you add another user to Confluence, making it 501 users to exceeding the user count limit. As such, no users will be able to save new pages, edit content or modify anything into it even though users can still log in to Confluence.

Solution

If you exceed the user limit, you have two options moving forward:

1. Upgrade your current Confluence license to a higher user tier. Upgrade pricing can be viewed here. 
2. Reduce the number of users with the Confluence 'Can Use', 'Confluence Administrators' or 'Confluence System Administrators' global permissions. You can go about this one of two ways:
   
   1. Deactivate the user
      We recommend that you deactivate a user's account vs. deleting the user. The reason being that the user's history of activity in Confluence will also be preserved. When you deactivate a user's account it will prevent anyone from being able to log in to Confluence using that account and the user will no longer count towards your licenses user limit. 
   2. Delete the user