The user count for your JIRA instance is calculated based on the number of users with permission to log in, i.e. users with the 'JIRA Users' global permission. When you hit your user limit you will be allowed to create new users.  However, these new users will be created without the 'JIRA Users' global permission and will be unable to log into your JIRA instance.

You have two options moving forward:

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

If your license user limit has been reached and you try and create a new user you will receive the following warning: