Scaling Push Messaging for Millions of Devices @Netflix (2019) talk is interesting. They can get 80k concurrent websocket clients on a single 8GB, 2vCPU server using an event driven architecture (I.e. without a DB on the same servers).
Nostr websocket clients are likely a bit more active in data, however 30-50k is likely achievable. It’s a pretty nice price point too, as 50k active users on a server that could cost $65/month (base cost), means Nostr growth of around 100x from today.
Some Nostr relay dev tips in there too, like asking clients to close the websocket gracefully so TCP CLOSE WAIT sits with the client (initiator) and frees up server file descriptors faster.