How to listen for multiple tcp connection using nc

I recommend socat as nc alternative.

For OP's problem, socat - TCP-LISTEN:12345,fork,reuseaddr can do the job.

ncat can do it.

E.g. ncat --broker --listen -p 12345 will distribute all incoming messages to all other clients (think of it as a hub).

Simultaneous connections are not possible with netcat. You should use something like ucspi-tcp's tcpserver tool or leverage xinetd since you're on Linux.

See: https://superuser.com/questions/232747/netcat-as-a-multithread-server

Consecutive connections could be handled through a shell script that restarts netcat after it finishes.