Hi,
Enterprise Cloud is currently in preview and is not GA yet.
In theory, there is no limit to the number of Studio licenses you can purchase.
If you request more licenses, sales will contact you.
Concurrent/Named User are the runtime types in view usage.
Unattended robots can run steps in parallel and the applications they run can utilize multithreading, but they are not truly multithreaded applications since they are not designed to run multiple processes in parallel on the same robot at the same time.
Studio Robot (2019.10) or Development Robot (2019.4 or older) is designed to work directly with processes that are written in Studio for immediate testing. Attended Robots run on someone’s workstation and processes are triggered on demand. Unattended Robots are scheduled via Orchestrator or are always listening for some kind of input and usually run on a server in a data center.
I hope this answers your questions.