Peertube uses bittorrent tech underneath to help distribute the load. Each viewer shares what they’ve downloaded to others viewing the same video at the same time. If 100 people are watching the same video at the roughly the same time the original host really only sends out 1 or 2 streams.

Things might get interesting if each of those 100 people want to watch a different video though. Pretty sure a server run by a popular youtuber like Mumbo Jumbo or even Justin Guitar would choke.