If you plan to just republish incoming WebRTC streams as RTMP to third party servers, without any subscribers, you should consider at least two servers for publishing. It should be at least 64 CPU cores and at lease 64 Gb RAM.
You can also place server instances behind a load balancer. Please read the example of AWS load balancer setup.
RTMP republishing may require video transcoding if WebRTC stream is published as VP8 for example. In this case you need more CPU cores, 1 core per 2 720p streams approximately.