Should a User no longer be employed, their account should be marked as "Inactive" rather than being removed. The reason for this is to retain the previous Employee's details and previous costs that are still tracked on previous Jobs/Call-Out's. When you mark a User account as "Inactive" the system will automatically make that Users slot available in order to add another User.
You can view all "Inactive" user accounts by un-ticking the "Active" box shown below