Ports like FTP and DHCP have two numbers because as we know the data communication can be allowed when there is a secured connection(not necessarily) between two computers ie a client and a server.
So FTP has two ports. Ports 20 & 21.
Port no 20 is used to pass control information. The data transmission deos not take place on this port number.
Likewise, port 21 is used for data transmission.
Hence the requirement of two ports.