Another thought I've just had, somewhat inspired by the Mayhem 21 issue - the general trend with newer levels is to usually keep the number of lemmings relatively low; whereas 80 or 100 was the usual amount in the past, these days 20 to 40 seems to be far more common. Should we make such changes to levels in this pack, too?
One possible reason against it is that unlike other NL trends, such as the removal of time limits and empty space, this change more often than not seems to not get applied to older levels when being updated for new versions of NL. (On the contrary, Lix does seem to be implementing this when updating, however one crucial difference here is that Lix no longer allows the player to change the release rate, while NL does unless the individual level prohibits it - which older levels won't, due to the feature not existing at that time.)
From a look through various recent NL packs, the general trend in NL seems to be towards 50, but levels with even fewer are starting to become more common very recently - for example, 20 is nearly twice as common as 50 in LPO2, and 30 is also more common than 50). And a point of interest is that Lix is usually a few steps ahead of NL in terms of level design conventions, and in Lix, the most common values (at least based off the LemForums pack) are 20 and 10. I don't know how I feel about using 10 as a standard, but if we do go ahead with this idea, using 20 as a "default" value should work well. Of course, this doesn't at all mean that no level can have a larger amount of lemmings - for levels such as Cascade that need the high number of lemmings to keep the original feel, there's no reason why a higher number can't be used, but I'm just meaning as a general guideline when there's no real reason to have more. (The Sega Master System version of Lemmings also used 20 as a maximum, and only a very small number of levels were worse off because of it. In the case of an NL pack, we can just not apply such a change to levels that would be affected in such a way.)