I am a novice of VDI. I would like to know whether VDI requires the purchase of windows CALS license?
About 20 win10 desktops.
I would start here. https://docs.microsoft.com/en-us/answers/storage/temp/12620-microsoft-vdi-and-vda-faq-v3-0.pdf
It depends on what type of device is connection to the VDI machines.
Talk to your reseller and they should be able to help with VDA licenses.