I don't know why it gets to such high COM port numbers. I use Teensys a lot and usually once a Teensy has been assigned a port, it will be that port whenever I plug it in again. But sometimes the system decides to change it and what used to be COM6 is now COM18. Same thing happened when I was using NANO, although not as much because I didn't have as many of them.
Occasionally, I open Device Manager, look at the list of COM ports, including the hidden ones that aren't currently plugged in, delete them all, reboot and then start plugging them in again to get lower COM port numbers.
Pete