"mononeuropathy" just means that one nerve tract is compromised, and "polyneuropathy" means that many are.

By the way, I've seen references to "multiple mononeuropathies", whichmenas that several individual nerve tracts are involved, as opposed to the more "global" dysfunction of "polyneuropathy". This "multiple" term is often used when some condition, often compressive, affects several nerve tracts in different parts of the body, but when the effect is not generalized all around the body.

One can certainly have a "polyneuropathy" but have one side of the body show worse symptoms that the other, or one part show the worst symptoms, but have some effects in other parts.

Ultimately, I don't find these terms very useful. I'm much more interested in whether a neuropathy is sensory or motor, demyelinating or axonal (although obviously they can be mixed), and, of course, what the cause is if that can be found.