I am an expert in Jitsi and Big Blue Button.
Can share my portfolio by email and answer all your questions. Also can help with setting up Jitsi, customizing it and integrating to your existing project.
About your questions:
1.As soon as you will create separate room for each interviewer, you can create almost as much as needed, that could be only limited by your hardware resources. As with growth of number of simultaneous calls, you will have increase of hardware and traffic ussage.
2-3.You can start with Digital ocean, $20/month can be ok at the beginning, then you can scale and upgrade it. Having own servers will increase cost, as you will need to maintain them too. You also can consider AWS.
My email is: firstname.lastname@example.org.
Let’s have a talk and I will estimate a cost of development, if it would be acceptable, can start As soon as needed.