The BIS_fnc_spawnGroup function does not work with a number as 3rd parameter, that specifies how many units (of random type) you want.
I tried to debug what happens but I did not understand the function 100%. But my guess is, that it is not able to pick some random unit types on its own and therefore spawns nothing.
The description of the function states for the 3 parameter
_this select 2: can be three different types:
- list of character types (Array)
- amount of characters (Number)
- CfgGroups entry (Config)
This goes for BETA and DEV branch aswell