Answer to #1:
The maximum number of users that can be simultaneously sending video to each other is limited by the capabilities of the hardware to encode and decode video streams. There is no hard limit.
If you are looking to do a single sender and multiple receivers, then again you are limited by the local machine. The sender will need to encode a separate stream for each receiver since the available bandwidth to each receiver will be different and impact the quality of video that can be sent.
Answer to #2:
You do not have to send audio and video. Even if you give permission to access your camera and microphone, you can later mute them (https://vline.com/developer/docs/vline.js/vline.MediaStream)
Also, take a look at this page for some more thoughts on this:https://www.webrtc-experiment.com/webrtc-broadcasting/
solved Maximum number of users in room