Delete the users can affect the records that they are being referenced in, therefore, it is might not be a good idea, we recommended that you make the users inactive instead. You would need to just remove the access role for the users you no longer need, this will them inactive in the system. Since we don’t count inactive users on your license, so you should be able to add more users once you have made a couple of users inactive.
Steps to make a user inactive:
On the Admin page, Click on User and Role Management.
Search the user that you want to make inactive and click on the pencil/edit icon.
On the Edit User page that opens up, un check all the roles under the Roles header and Save the record.
On the User Management page you would not see the status of the user as Inactive.
Or if a user leaves and you can't delete them because they have been referenced in a record (E.g. had a task assigned to them) then yoy can simply remove their Role and they can't access the system anymore.