Server video mixing recommendations?

I see a lot of docs about how Jitsi’s videobridge does not mix video, it only forwards video.

What if I want to mix video on the server? Because I want to mix 100 or 1,000 video streams into a single stream, and I assume this is too many separate streams for clients to handle, but a sufficient cloud machine (or a handful of them in a tree configuration) could handle it.

Any recommendations for open source projects or frameworks for doing this?