Whenever a new USB serial device is plugged into a PC, Windows will assign the device a new, unique COM Port number. This could even happen if the same device is plugged into a different USB port. I don't mind a few unused ports, but COM20 is a bit ridiculous.
Here's how to fix it.
Open a CMD prompt then enter these commands:
C:\> set devmgr_show_nonpresent_devices=1 C:\> start devmgmt.msc
In Device Manager, click View, then Show Hidden Devices.
Expand the section labeled Ports (COM & LPT). The light grey icons are unused ports. Select the port, press DEL.