I feel it's currently in beta, I've tried it and apparently I can't create more than a few instances because my account is "too new", without a clear way to remove that limit so you're right, can't have a large bill if you can't even create 10 instances.
Did you try writing them and asking them to increase the limit?
No cloud provider will give you the option to create as many instances as there are available ones, they all have limits from the get-go. Usually you have to write them/fill out some form if you want to go above the standard limit, Hetzner Cloud as well.
I haven't, mainly because of this warning on their Limits page:
> Your account is too new to request a limit increase. Please note that we generally do not answer questions regarding limit increase on the telephone.