Personally on my coop server there is a need for 32 slots (25 players + 6 bots + additional space for hidden bots)
In the end, you yourself can change the maxplayers to any number you want in the plugin source code)
The problem of additional campaigns is the problem of the authors who created them like this) On standard campaigns and on City17, Urban Flight and Suicide Blitz 2 I tested, everything is fine with maxplayers 32.
I only had crashes on the c6m1_riverbank map (when real players more 16). Removing debris from the map through a stripper helped solve the problem of crashes due to the increased number of players