A wildlife expert predicts that the reintroduction of the caribou into the northern Minnesota would fail if the density of the timber wolf population in that region is more than one wolf for every 39 square miles.

I understand that density is countable so we should use greater than, but in the flash cards I find this "greater than, More than" concept: "More than should be used when describing the numbers of objects" and " Greater than should be used when describing numbers alone" -- going be this i feel we should use More than in the sentence above because this sentence is about the wolf ( or the object ).

Please help me out with this confusion..

Naveen