When (if ever) to use plural form of Homo sapiens?
Solution 1:
Never. If you really want to use Homo sapiens, you should say
One Homo sapiens
Two Homo sapiens
Three Homo sapiens
Four Homo sapiens
Too many Homo sapiens
This is the same for all species names. For example, you would say five Mus musculus or six Tribolium castaneum. As a general rule though, you just wouldn't. Even in scientific literature, you will tend to refer to the species by their common names, so four humans, five mice, six red flour beetles.
Solution 2:
You should say:
These are two fine examples of Homo sapiens.