From curseforge:

Where in the config file are you able to change spawn rates for particular mobs (Skeletons, Zombies, Creepers and Spiders etc.) I only ask because i'm using this mod on a hosted server and with the amount of people playing i'd like to lower the spawn rates because it seems that rough mob spawn rates are higher than regular vanilla mob spawn rates.

All spawns are vanilla. RMR simply adds gear and abilities to vanilla spawns. There is no change to spawn rates. Try using something like Incontrol! Mod. It works great for controlling spawn rates.