The way I increased the zombies on my server was to go to C:\Program Files (x86)\Steam\steamapps\common\DayZServer\mpmissions\dayzOffline.chernarusplus\db
Find Events. Open it with Notepad++.
Go down to InfectedArmy and increase the Nominal, Min, and Max counts.
Here is what mine looks like :
<event name="InfectedArmy">
<waves>0</waves>
<nominal>100</nominal>
<min>75</min>
<max>300</max>
<lifetime>180</lifetime>
<restock>0</restock>
<saferadius>100</saferadius>
<distanceradius>50</distanceradius>
<cleanupradius>100</cleanupradius>
<flags deletable="0" init_random="0" remove_damaged="1" sec_spawner="0"/>
<position>player</position>
<limit>custom</limit>
<active>1</active>
Directly under that, you will see The children of this category. Raise the Min count on those as well. Mine looks like this :
<children>
<child lootmax="5" lootmin="0" max="0" min="40" type="ZmbM_PatrolNormal_Autumn"/>
<child lootmax="5" lootmin="0" max="0" min="10" type="ZmbM_PatrolNormal_Flat"/>
<child lootmax="5" lootmin="0" max="0" min="20" type="ZmbM_PatrolNormal_PautRev"/>
<child lootmax="5" lootmin="0" max="0" min="10" type="ZmbM_PatrolNormal_Summer"/>
<child lootmax="5" lootmin="0" max="0" min="20" type="ZmbM_SoldierNormal"/>
Do this for InfectedCity, InfectedFirefighter, InfectedMedic and so on.
Then open Globals. Increase the ZombieMaxCount numbers to your liking.
Enjoy.