I previously was involved in a service where WebRTC was used for a fairly large number of N streams to M viewers. Like ~75 streams total and 500+ viewers. Solution space was public sector incident command (think police officers with multiple cameras on their body streaming to command center).

Microsoft's now shut down Mixer (formerly Beam.pro) primarly used WebRTC and had streams with up to 100k viewers at some point all via WebRTC, but can't find much info about it anymore now sadly.