Recently looking at some charity licensing where I found 2 sets of Windows Server User Cals

https://www.connectingup.org/donations/content/microsoft-windows-server-user-cal-includes-software-assurance ( SKU: LS-40627 ) 

https://www.connectingup.org/discount/microsoft-discount-volume-licence-program/microsoft-windows-server-user-cal-licence-only-0?action ( SKU: R18-05766 ) 

The first one was nearly half the cost of the other , turns out SKU: LS-40627 you have a limit of 50, so you will need to purchase these before purchasing anymore of R18-05766  ( Can be purchased in parallel )