I can see that the 1-in-20 rule is a botanical rule, because it would be
difficult to apply it to most animals, including snails, especially if they
cryptic & micro species. Plants don't hide, animals do. There may be 1000
specimens of a certain micro snail in a patch of forest litter, but it could
take one a whole day to find a single specimen.

A more sensible, but more vague rule would be to collect specimens of a
species if one "keeps seeing it". For example, Neohelix albolabris is large &
common eastern U.S. land snail. In the woods where I routinely collect, I have
never seen more than one or two over a course of 2-3 hours of collecting. But,
I come across them very frequently. So, in this case, collecting a live
specimen every now and then shouldn't hurt the population.

A.