It is always per system (whether that system is a virtual machine or “VPS” or bare metal is irrelevant), and sized based on number of domains being hosted.
When i buy pro license, it says select domain (10-50-100-250-Unlimited)
So if i buy 10 domain license, means I can host upto 10 different website/domains, irrelevant if all of them are on one instance or spread across 5 instances ?