From https://social.technet.microsoft.com/Forums/ie/en-US/eac06eab-3455-4d8e-8698-ddb8d4189e64/reducing-the-number-of-remote-desktop-licensing-peruser-cals-?forum=winserverTS

We can use the following commands

Get-WmiObject Win32_TSLicenseKeyPack|Select KeyPackId, ProductVersion, TotalLicenses, TypeAndModel|Format-List

Invoke-WmiMethod -Class Win32_TSLicenseKeyPack -Name RemoveLicensesWithIdCount -ArgumentList <Keypack>,<NumberofLicensesToRemove>

For Example, if we had 14 RDS cals, and wanted to reduce them to 12:

PS C:\Windows\system32> Get-WmiObject Win32_TSLicenseKeyPack|Select KeyPackId, ProductVersion, TotalLicenses, TypeAndModel|Format-List

KeyPackId : 2
ProductVersion : Windows 2000 Server
TotalLicenses : 4294967295
TypeAndModel : Built-in TS Per Device CAL

KeyPackId : 3
ProductVersion : Windows Server 2019
TotalLicenses : 14
TypeAndModel : RDS Per User CAL

You can see KeyPackId 3 has the RDS cals we want to reduce, we would use the command:

Invoke-WmiMethod -Class Win32_TSLicenseKeyPack -Name RemoveLicensesWithIdCount -ArgumentList 3,2

This would reduce Keypack 3 by 2 licenses, bringing it to a total of 12.