In this country, I’d say between 2-6 million depending on how liberal we want to be with definitions. About 1.5 million identify as actively practicing Wiccan, Pagan, or Heathen, and given the number of places where it’s not safe to admit to something like that, we have to assume there’s at least a few more- I’d peg the real number at around 2.25 million.
Then we’re looking at the stuff that’s adjacent, whose followers wouldn’t call themselves “Pagan”, but which still fall under the polytheistic-ritualistic-naturalistic triad that kinda defines the Pagan/Heathen umbrella. That includes a lot of the New Agey stuff, but also things like Hinduism, African diasporic Voodoo and Palo, Native American beliefs, and “old world” holdover traditions. If we’re lumping all of that in, we’re going to need to double, maybe triple that 2.25 number.