I made a post some time back about maintaining 64k connections in a Ruby process a while back: https://www.wjwh.eu/posts/2018-10-29-double-hijack.html . It only stopped at 64k because I could not be bothered to rig up multiple IPs so it eventually ran out of ports.

Just having a lot of connections that do trivial stuff is not very difficult. It becomes way more interesting when all of those connections need to access shared data structures and whatnot.