Azure Stack HCI is a virtualisation platform for Windows and Linux virtual machines, and customers have a choice as to how they license the Windows part of the VMs. Firstly, they can bring their own licences, and there’s no Software Assurance required when Azure Stack HCI is running as an on-premises solution dedicated to the customer.
From 1 April 2022 there’s a new option with the launch of special Windows Server Subscriptions specifically to license the guest (VM) operating systems on Azure Stack HCI. As is usual with Windows Server, you license at the physical core level, and a Windows Server Subscription costs $23.30 per physical core per month, with the first 60 days free.
Find the announcement here: https://bit.ly/3LrZXZd, and a useful page comparing the Windows Server licensing options for Azure Stack HCI here: https://bit.ly/3EW9zcz.