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.

That's it!