how many systems can I have in one account?

Hello
This question boggles me and I need your help please

I will be setting up several customers devices (side question, when registering a device do I use my information and email or do I use the customer's)?

Now I want to manage several systems and connect to them with myhs, can I register more than one system with the same email and password for my access?

Or does each system has to have its own and I will require to have a different email for each setup I make?