|Because with ftp, one is used for control and the other data transfer. Just think of it as a single port because that's how it behaves.|
Just FYI, if you had googled for "ftp port number" and done a little reading, you could have easily discovered this answer all on your own and saved us some time and effort.
Remember, Google is your friend
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.