Hi Aleksander,
Let me start off by saying I am not a licensing expert. But what I do know is that to enable Azure Hybrid Benefit, first you must have a valid Software Assurance agreement. Then, by activating Azure Hybrid Benefit, you are agreeing to reserve the equivalent number of SA core licenses for the cores within your Azure Stack HCI cluster. In doing so, you cannot use those SA core licenses for any other purpose and you will not be billed for the cores within your Azure Stack HCI cluster.
As a very simple example, assume you had a 4-node cluster and each cluster node had 32 processor cores. With Azure Hybrid Benefit, you would reserve 128 SA core licenses to cover the 128 cores in your Azure Stack HCI cluster. In return, you would not incur the core charges for that cluster. Without Azure Hybrid Benefit, you would be charged at a rate equivalent to $10USD/core/month. Note that Azure Stack HCI clusters upload billing information to Azure twice per day, so your bill will reflect the daily charges which average right at $0.3333USD/core/day. So for a 128-core cluster, you would be billed approximately $42.66USD/day or $1280USD/month. More pricing details (including different currency rates) can be found here - https://azure.microsoft.com/en-us/pricing/details/azure-stack/hci/.
Note that other services/features that are added to the cluster may incur charges, so be sure to review the terms for each additional service before adding it to the cluster. An example of this is AKS. (As a side note, Azure Hybrid Benefit is also available for AKS - https://video2.skills-academy.com/en-us/azure/aks/hybrid/azure-hybrid-benefit?tabs=powershell).
As for the subscriptions, there are several options available. You can use our pricing calculator (https://azure.microsoft.com/en-us/pricing/calculator/) to get an estimate based on your needs. However, I would recommend that you reach out to our Sales team or even one of our partners so they can help ensure you get the right option.
Hope this helps!