thanks for your answer KhaineGB
I haven't found the parameter for the random range in blocks.xml
<!-- Wandering Horde Config -->
<block name="WHConfigBlock">
<property name="Extends" value="terrDesertGround"/>
<property class="WHConfigLoad">
<property name="Logging" value="false"/>
<property name="WHHours" value="6" /> <!-- How many hours between wandering horde spawn -->
<property name="WHZombies" value="40" /> <!-- Amount of zombies per horde -->
</property>
</block>