Is the Number Appearing entry based on the stats of the monsters?

Question

For humanoid monsters in the Basic and Expert sets there is a clear pattern of creatures with the same number of Hit Dice appearing in smaller numbers in random encounters if they have better armor and damage, and in larger numbers if they have lower armor and damage. This pattern continues to simple predatory animals like wolves, boars, and bears.

Among higher HD creatures there seems to be a trend towards smaller groups as these tend to have additional special abilities beyond just direct damage attacks and only armor for protection.

Is this simply something that was eyeballed by the writers when having comparable creatures with the same number of HD, or is there any kind of measure by which a fitting group size for a creature of a given HD value can be judged when creating new custom monsters?

Answer 1

There's no exact measure, but in both Moldvay's Basic and Cook's Expert set, the text makes it clear that the general principle is that of balance:

The Wandering Monster tables (below) give a balanced mixture of monsters for the dungeon levels.

And in Moldvay we also find a specific reference to what this balance might look like:

Most Wandering Monsters are the same level as the level of the dungeon (...) The "Number Appearing" of some monsters has been adjusted to make them more appropriate for encounters on a dungeon level.

While there is no specific information regarding how this works in terms of the armor and weapons of creatures of the same hit dice, it is reasonable to assume that the same concern for balance motivated the adjustments.