Maximum Concurrent Connections

We recently completed a Poker game built on top of Colyseus. It has a lobby which is a Colyseus room. From there users can select tables which are separate rooms.

We have now started doing simple load test, and are struggling to go beyond 3500 concurrent users. The load test is very simple. The client connects to the lobby, and stays idle. There is no processing.

We have adjusted the max file limit (500000 soft/hard). However, 3500 concurrent connections seems like the maximum we can push. The CPU idles around 15%usage with spikes of upto 50% and more than 50% of the ram is unused (no swap).

Is there a hard limit on number of connections per room? Has anyone had success with a higher count?

I did not have the experience and the need to support this amount of clients but what about splitting the server into different instances?