Mobs spawning too much frequently

I want to make a bunch of mobs that spawns in different dimension biomes so that they can populate a bit the dimension. I want them to spawn in normal proportions, at a zombie/creeper level, but everytime I get the mob spammed all around the specific dimension biome, with 5+ entities per chunk! They also spawns in groups of 2-5, but I set they can spawn in groups of 1 max! I tried editing the spawn rate (100-20-15-10-1) but the spawn rate barely changes. I tried to edit the code (making them in groups of 0/1 instead of 1/1 because Mcreator won't let you put a number smaller than 1), but everytime I try it doesn't changes. Can somebody help me? Maybe with some advice for how to change the code that changes the spawnrate?