The Monster Manual (1st Edition) indicates the number of monsters encountered and the treasure type. In general, I like these guidelines.
But say for instance, I'm rolling for number of troglodytes which could be anywhere from 10 to 100. And the treasure type is listed as 'A'.
If I get very low numbers on the roll (like 20 or 30), does it make sense to still use the 'A' treasure type?
Yes, but with adjustments.
First, the treasure type doesn't mean there will always be treasure. It's entirely possible to come up with nothing after rolling for Treasure Type A. Second, once you've determined the hoard, it should be adjusted by your best judgement if there are many or few creatures.
On sometimes rolling up no treasure at all, from the Monster Manual, page 5:
Note that although an encounter occurs in a monster's lair, and the monster possesses some treasure type, this does not automatically mean that the adventurers will gain treasure […] If subsequent dice rolls indicate that that form of treasure is not in the monster's trove, then it is not there, and it is quite possible to come up with no wealth […] of any sort in a monster's lair despite the fact that a treasure type is indicated.
On adjusting the treasure based on unusually small or large groups, same section:
[T]reasure types are based upon the occurrence of a mean number of monsters as indicated by the number appearing and adjustments detailed in the explanatory material […] Adjustments downward should always be made for instances where a few monsters are encountered. Similarly, a minor adjustment upwards might be called for if the actual number of monsters encountered is greatly in excess of the mean.
In general, the "Explanatory Notes" section at the beginning of the Monster Manual is a great source to consult when trying to understand the whys and hows of early-edition D&D play. A lot of the assumptions that the system is built on can be extracted from these sorts of out-of-the-way places in the texts.
It does make sense to use treasure type A. Usually there is some % number there that indicates the percentage chance of a particular type treasure being present. It also indicates amount of treasure/creature present.
I often do something other than what is indicated in the MM. In general, I look at the number encountered, consider where it is these creatures are encountered, consider what it is I'm trying to accomplish in placing a monster in a particular place and then decide on how many are there and what sort of treasure they have. As long as you stay internally consistent, it seems to work out well.
