When you try to install the additional SBS client licenses, you receive:
Your computer already has the maximum number of licenses. You cannot add any more licenses to your computer.
but you have less than 21 licenses and the maximum is 25.
Setup checks the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LicenseInfo value. If this REG_DWORD value is more than 20 (hex 14), the install will fail. Use Regedt32 to double click the LicenseInfo value name and set the data value (in decimal) to the number of licenses in the About Microsoft BackOffice Small Business Server dialog box.
Use Control Panel / Services to browse to the License Logging Service. Select it and press Stop. Once it has stopped, Start it and add your new licenses.
Someone probably changed the AutoUsers= line in the \[LicenseFilePrintData\] section of the Winnt.sif file prior to installation or was mucking around in the registry.