thats in monster.conf, not sure if you can change it via scripts.. maybe you can do it with:
http://rathena.org/wiki/Setbattleflag
like: SetBattleFlag "mob_count_rate", 200; for double spawns
Hmm, the picklog also logs every kill and drop, isnt it the same?
why just dont make a script for reading the picklog file, if sql logs are enabled its saved into sql also.
... oh well, the picklog only logs items with their id / map / amount.