I have no idea (maybe PHP). I just saw that they a short info text in the backend and display a warning when you are having a conversation with more than 4 participants. Couldn't find any documentation on what the underlying problem is.
> An external signaling server should optionally be used for larger installations. Leave empty to use the internal signaling server.
> Please note that calls with more than 4 participants without external signaling server, participants can experience connectivity issues and cause high load on participating devices.
Probably it's just the sheer amount of connections you have to make. But it's possible to have even 10 people. Especially if you go through a central hub.
