Microsoft KB 888818 reports that it is a KNOWN BUG that SBS2003 sometimes resets to the default of 5 Client Access Licenses (CALs). This can be caused by a number of different circumstances. They propose a handful of different solutions:
Solution 1: Restore CALs using the Backup and Restore CALs wizard in the licensing manager.
Solution 2: Restore the Windows folder from a Backup.
Solution 3: Re-add the CALs to the licensing manager from the original paperwork.