This is just from my understanding. It may not be 100%
I think the serverconfig.xml will only affect an actual dedicated server. If I am wrong someone please let me know also as I would love to know this for certain.
I also recall there are some mods out there that will help set the amount of zombies in game.
I have also seen you can set the max amount higher in the windows.xml file in the config/XUi_Menu folder...
This is in the main games folder... .../Data/Config/XUi_Menu/windows.xml
can do it by adding numbers...
make sure you follow the pattern of the line, no left out or extra spaces, commas where needed etc. And always make a copy of the file before editing in case of a screw up.
you will also see that in that section there are other things that can be edited for the game menus.
I wouldn't go crazy with really high numbers until you see how smaller changes work out.
Again...VERY IMPORTANT...back up files before editing...can't be said enough.
Have Been Warned.
(and welcome to modding)