Difficulties

Each difficulty increase in The Alternate Difficulties mod typically increases the amount of zombies. This is achieved in many ways. The mod increases the frequency of mob spawns but it also reduces the amount of zombies that will spawn in each mob. Since wander zombies can migrate from mob spawns its an easy way to increase the number of wanderer zombies the players will encounter.

This means that if a mob of 20 spawn there might be 10 that are set aside to pad out the wandering zombies (gauntlets excluded). The players get 10 of 20 zombies in a hoard then the following 10 a bit afterwards and the wanderers will be refreshed.

The higher the difficulty, the smaller the hoard but the more frequent the mobs. Resulting in more wanderer's and more kills per map.

Increased difficulties will also increase the damage taken by zombies but that exact number will depend on the tier of difficulty you are currently in.