This function will count all the monsters on the specified map that have a given
event label and return the number or 0 if it can't find any. Naturally, only
monsters spawned with 'monster' and 'areamonster' script commands can be like
this.

However, apparently, if you pass this function an empty string for the event
label, it should return the total count of normal permanently respawning
monsters instead. With the current dynamic mobs system, where mobs are not kept
in memory for maps with no actual people playing on them, this will return a 0
for any such map.