By way of review, All network transactions happen between clients and servers. In most protocols , the clients ask a certain address and receive data.
Within each address, a multitude of servers can run. The limit is in the hardware. With sufficient hardware (RAM, processor speed, etc.), the same computer can serve as a web server, an ftp server, and mail server (pop, smtp, imap, or all of the above) all at the same time. Each service is associeted with a port. The port is bound to a socket. The server listens to its associated port and gives information when requests are received on that port.