It's port 20
according to the wiki.
Port 21
is used for control data. The actual file data is sent on a different port.
To my knowledge, that port is usually negotiated before transfer. I don't know if Wireshark uses a fixed value of 20
or if it knows what port was used for data transmission.
Please note that what Wireshark calls a protocol is something like TCP.
I'm assuming you're talking about a human-readable version of a port.
From: http://www.wireshark.org/docs/wsug_html_chunked/ChAdvNameResolutionSection.html
7.7.5. TCP/UDP port name resolution (transport layer)
Try to resolve a TCP/UDP port (e.g. 80) to something more "human readable".
TCP/UDP port conversion (system service): Wireshark will ask the operating system to convert a TCP or UDP port to its well known name (e.g. 80 → http).
XXX - mention the role of the /etc/services file (but don't forget the files and folders section)!
So, it's a static lookup approach from port number to service. The information is basically meaningless.