sorry not OP.

How do you configure multiple servers to work?

I plan to play trought the maps in order and I was able to configure the first 6 free maps.

I can't find a list of allowed ports to use.

As a test I started them one by one, each of them using their own ports

7777 | 7778 | 27015
7779 | 7780 | 27016
7781 | 7782 | 27017
7783 | 7784 | 27018
7785 | 7786 | 27019
7787 | 7788 | 27020

but the next ports don't work:

7789 | 7790 | 27021

I'm playing on my own desktop in LAN so - in theory - I don't need them to show up,

but I noticed that you can't transfer between my servers if they are "offline" aka not showing up on Steam

(and I can't play with my friends)

I don't think that manually enabling 5 ports in my router, re-configuring the server, starting the server, waiting a few minutes is the correct way to do this. I'm not a brutforce tool :P