If the DM decides that yes, this encounter should happen, he determines how
many creatures or NPCs appear. There is no quick and easy formula for this.
Experience is the best guide. The Monstrous Compendium lists a typical encounter size for each monster. Use this as a guideline,
especially when you're first starting out as a DM, but don't follow this
When uncertain, use a small encounter. It is far better for a random encounter
to be easily defeated by the player characters than it is for the monster to
overwhelm them. An easy PC victory gives the DM information and experience (so
he'll know to increase the difficulty of the next encounter) without harming the
player characters and his campaign. A crushing PC defeat is almost impossible
to correct without obvious manipulation once the encounter has begun.
As always, use common sense when determining how big an encounter is. Nature
provides some guidelines. Bear these in mind when figuring encounter size.
Many predators, especially those that hunt by night, are solitary creatures. A
nocturnal fantasy creature might show up alone, as well.
Of the predators that hunt by daylight, some work alone while others cooperate
in groups of two or three. One or two will attack the prey from one direction
while the others wait for it to be flushed toward them. Such hunters are
usually stronger and faster than their prey. Again, fantasy creatures can follow this
Smaller predators sometimes hunt in packs of 5 to 12, attempting to surround
and harry a chosen victim. Herbivorous animals tend to flavor herds and the
company of others. Omnivores live in smaller groups and often have older members
that act as guards. All of these factors can play a part in the size of a given
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