Addon: Too Many Mobs

With this addon mobs have double spawning, for more context that's double weight and double the spawn groups. To explain that, weight is how likely a mob is to spawn and group is the sizes of groups mobs can spawn in.

Created By SUBERNER STAR

So now, zombie can spawn in groups of 8, creepers can spawn in groups of 2, skeletons can spawn in groups of 4, Ghast can spawn in groups of 2, frogs can spawn in groups of 10, and all other mobs will also increase in spawn sizes.

It addon isn't extreme to the point were there monsters everywhere you are, mobs just spawn in bigger groups and spawn more often, however this can still make the game much harder.