here..
*getmapmobs("<map name>")
This function will return the total count of monsters currently located on the
specified map. If the map name is given as "this", the map the invoking
character is on will be used. If the map is not found, or the invoker is not a
character while the map is "this", it will return -1.
perhaps it is removed in rAthena...coz i ddint found it in
rathena/trunk/doc/script_commands.txt