Numerous refers to a collection that is great in number. I mentioned in my answer on the deleted question the example of numerous rabbits, which can be read as a collection of rabbits that is great in number (or something similar - I don't have enough rep to view it).
It seems to me that the word numerous only applies to collections with countable, distinct elements. So the usage in this case is wrong because a single name does not qualify as a collection at all; it's simply the same name being used numerous times (times here is a collection). You can say
Buckley is the 99th most numerous family in Ireland
People named Buckley are the 99th most numerous group in Ireland
because here, family and people named Buckly are both collections of individual elements which can be counted (or numbered).
However, leeches is a collection which can be great in number. In particular, Helobdeila stagnalis is the sub-set of all leeches which is greater in number than any other sub-set (by species) within Colorado.