There is no limit in number of streams, other than the hardware spec. For 400 streams, you should use at least 2 servers (I’d use maybe 4), although it’s possible to run it on a single powerful server.
Single Server: 2x Xeon Gold with >12 cores, and configure the heap size to 24-32GB. Disable monitoring.
Two servers: 2x Xeon Silver 10 cores, or 1x Gold with ~16 cores, and a heap size of 16GB
Four servers: Xeon Silver with 8-10 cores and default heap size as recommended by Wowza (10GB)
That’s just my recommendation based on some experience, without any further guarantees. It’s not an official recommendation from Wowza