Number of Wandering Monsters Appearing: If the level beneath the surface roughly corresponds with the level of the monster then the number of monsters will be based on a single creature, modified by type (that is Orcs and the like will be in groups) and the number of adventurers in the party. A party of from 1-3 would draw the basic number of monsters, 4-6 would bring about twice as many, and so on. The referee is advised to exercise his discretion in regard to exact determinations, for the number of variables is too great to make a hard and fast rule... [Vol-3, p. 11-12]I've posed this question before and got some good feedback. Probably the best suggestion was "if in doubt, roll 1d6". Let's expand on that just a bit.
The quote above makes a distinction for a "type... [that] will be in groups" -- I guess that would be the humanoid types in Vol-2 that potentially come in the hundreds in the wilderness? Anyway, I'm not sure the distinction makes sense. Let's say at level 1 you generate Skeletons or Spiders or Giant Rats or something. Would you really want this to be "based on a single creature", i.e., you just encounter Skeletons or Spiders or Rats one at a time? That seems quite seriously deflating.
Personally, I don't see any reason why you wouldn't also want to see these non-humanoids in a group (such as 1d6 in size), both generally for interest, and also for game-balance sake (an intent which is implied in several places in OD&D). These other level 1 creatures are not more powerful than Orcs; since they're all about equal danger level, you'd think they should appear in approximately the same group sizes. Extrapolate this further -- while 1 Ogre makes intuitive sense on level 1, once you're on level 4 I wouldn't expect to see solo Ogres by default, I'd expect to see a whole bunch of them (possibly "a street of masses of ogres", as on Greyhawk's 7th level).
Obviously, my intuition was not used in later editions, which did explicate numbers appearing in later Wandering Monster charts, and show the Gygaxian naturalistic inclination of big groups of humanoids (7-12 orcs, 6-15 goblins in DMG) and much smaller collections of other types (1-4 fire beetles, 1-4 skeletons) -- even though the latter are clearly weaker encounters overall. I think that's a mistake; but fortunately OD&D directly advises that the DM "exercise his discretion" on this point, so we have a lot more freedom here.
So let's keep it simple -- Make the "basic number" for monsters just 1d6 in all cases (excepting creatures listed in the singular). Modify this by level as follows: Multiply/divide by 2 cumulatively for each level that the dungeon exceeds/undervalues the monster level.
This is what I've started using recently, and it seems to work well in the tests that I've run. It sort of follows the 3E observation that a x2 increase in number is worth a +2 increase in challenge rating (borne out as pretty accurate when I tested it) -- although in OD&D the levels are more compressed (only 6 categories, e.g., one monster level spans two character levels of fighters/wizards), so it's looking to me like x2 numbers per +1 monster/dungeon level is most fitting. (As a side note, I'm also awarding XP on the same basis, starting at 50XP for a level 1 monster, 100XP for level 2, etc.)
The playtests that I've been doing with a slightly modified OD&D Monster Level Matrix and Wandering Monster Tables seem to work very well -- dangerous but conceivably survivable with a bit of luck for PCs of an appropriate level. In summary I currently recommend:
- Basic 1d6 of any monster per encounter (exceptions: Ochre Jelly, Hydra).
- Multiply or divide by 2 for each level the dungeon differs from the monster, i.e.:
- Multiply/divide by 2.
- Multiply/divide by 4.
- Multiply/divide by 8.
- Multiply/divide by 16
- Multiply/divide by 32.