When I try to increase the player limit in the settings, it tells me that I have to use some command line parameters. I found the “Techinfo.txt” file https://github.com/id-Software/Quake/blob/master/WinQuake/data/TECHINFO.TXT, but it is very old, confusingly technical and I doubt that it would apply to the modern engines.
So, how do I play a multiplayer game with more than three of my friends, with preferably the QuakeSpasm engine?
You could use Quakeworld instead of Quake to play a game with more player than 4 players. Setting up your own server is easy (http://tinyurl.com/z8o3csw) or you could use something prebuilt like http://www.quakeonlan.net (if you have a Raspberry Pi handy).