It begins really easy with the few numbers you find in your standard old school Monster Manual. Do you see this Number Appearing line on the picture above? It's about this. I wonder if many Dungeon Masters have actually used this number ever, and the long sections detailing humanoid monsters lairs, allies and structure of power. Surprisingly, most humanoid entries of Advanced Dungeons & Dragons include such details, appropriate to the most absurd figures dungeon-wise. Who can throw 300 orcs in a dungeon? Wouldn't the dungeon become a Bara-Dur fortress of sorts if we did?
I can think of 1 or 2 officially published adventures at least that did : the U3 module, The Final Enemy, the A2 maybe, Secrets of the Slaver's Stockade in a more devious way.
Most adventures make an extensive use of this Number Appearing logic, but they do it in a nutshell: a few gnolls here and there, some goblins — are they many enough to pour a shaman and a chieftain in? Cool. I can think of none who purposely used it in order to create and shape the adventure.
Well, I did. I rolled an orcish tribe and rolled about 250 orcs, discovering two « effects » as I did. Here they are:
1. The Domino Effect
The Monster Manual says orcs must have a strong leader, telling me there's maybe a wizard or an evil priest. Let's put that question aside for the moment, but keep it in mind. For about 250 orcs, I have 8 leaders and 24 assistants. I add flavor here and, inspiring myself upon the Lord of the Rings and my old Sword & Sorcery SPI wargame, I decide that these stockier, more powerful orcs all belong to a special sub-race : white orcs. In Swords & Sorcery, white orcs are followers of the Czar. Hey, why not?! I also get 21 bodyguards, fiercer orcs, cadets of the crown maybe — do they have an uniform? —, and I'm hinted at adding a few ogres. I do, of course. Now, going to the ogre section, I find that ogres often ally with gnoll raiders, trolls and stone giants. Okay, there's gnoll raiders too, then, and maybe a troll or two. Looking at the gnoll entry, I'm told that gnolls follow evil priests. That solves my first question, the tribe leader is an evil priest. The gnolls are also allied with trolls (I have them already) and a few hyenas. Great, I now have a kennel.
This is the domino effect: my dungeon is now stocked with standard orcs, white Czarist orcs, ogres, gnoll raiders, trolls and hyenas, all under the power and command of an evil priest. Since half-orcs are described under the orc entry as well, I add some, giving them character class levels, as lieutenants of the evil priest.
2. The « Stocking First » Effect
The monster section actually says a lot more: it says that orcish lairs might be above ground or underground. I roll, and get underground, which is nice because the same section tells me how good they are at mining and underground constructions. Orcish tribes also sport a name. Mine being led by an evil priest and Czarist orcs, I choose the Vile Rune, a name that tells of northern wastes, ancient primitive religion and evil. Under the gnoll section, I find that gnolls often live in abandoned villages. So here I am, in an underground mining lair with many slaves close to an abandoned village. Since there's a priest, there's a temple too, hidden inside, all bowing to the power of the orcish Czar.
Take a closer look: I have an adventure, fully-fleshed here. It's about freeing slaves, beginning in slavery maybe? It rings an Indiana Jone's Temple of Doom sort of bell in a Russian-like setting. I now need a god about which the evil priest's cult revolves, and so on... Are the player characters hired by the orcish revolution? Knights fighting the evil cult? Mercenaries of a border kingdom threatened by the Vile Rune?