Open scenario.xml and set where is like "monstercount=8" to "monstercount=32" or in npcset.xml found in the server set "maxnpcs=32" to "maxnpcs=xx"
In both you can change, and in npcset it's like general, every stage has the set (like Sa1, Ga1, Ga4, etc.) will have the rate of monsters rased. I hope I've helped xP