Count number of connections in java socket

A new connection is established when the accept() method called for the listening socket returns. The connection is closed when closing the socket returned by the accept() call. So you have to change the counter at those points in your code.

Basically:

socket client_socket = server_socket.accept();
conn_counter++;
// ...
client_socket.close();
conn_counter--;

If a server has to support multiple clients connected at the same time, it has to use an own thread for each connection. Such is handled by most socket tutorials like Introducing Threads in Socket Programming in Java - GeeksforGeeks[^].