Technically, there are many different definitions of pagan in use by the people, several of which have no requirement of polytheism. Some use the word pagan to refer to a non-believer or an atheist. Some use it to refer to anyone who isn't part of the Abrahamic faiths or some other major world religion. I tend to go by a more useful definition of Pagan with proper case, and even then I don't restrict it to polytheism. Paganism with a capital "P" has a pluralistic concept of the divine. That is to say, the ways of looking at deity within Paganism are many, and we don't tend to consider one point of view absolutely correct like you see with some exclusivist monotheistic faiths. The divine is usually acknowledged to be many, but actual practice and worship in Pagan religions can often look like monotheism (aka, henotheism).
Honestly, I'm tired of folks making it about a numbers game in the first place. If someone asks me if I'm a monotheist, I say yes. If they ask me I'm a polytheist, I say yes. When they give me a confused look, I say "that forest over there is composed of a bunch of trees; I can approach the forest or the individual trees within it. It is neither a forest NOR a bunch of trees. It is both and neither."