Pagan Population estimates: statistical data from CUNY, and analysis by Cec
While I hate to limit us in numbers, on the other hand these are the first numbers identified by statistically valid methodology by an independent entity and respected academic resource, so for publicity purposes they are valid.
Here's my analysis of the figures:
The first good independent survey (to our knowledge anyway) identifying numbers of self-identified Pagans in the US came out in late 2001. It's the American Religious Identification Study presented by the City University of New York. The raw data is at http://www.gc.cuny.edu/faculty/research_briefs/aris/key_findings.htm.
You can add Wicca, Druid, and Pagan together to get a total number. I also added 19% of the Unitarian Universalist figures because their 1998 National Conference survey found that 19% of their population considered themselves primarily "earth-religionists". This gives us over 425,000 Pagans in the US, or just a bit more than .2% of the population. That means that 1 in 500 people you meet is Pagan. Pretty good numbers when you consider that there still are a -lot- of people who won't self-identify as Pagan for fear of discrimination! [Personally, I think this number could be as much as 30 - 40% short based on that discrimination fear, but I have no hard data to back that up.]
The only Pagan category they had in their last survey in 1990 was Wicca, and there were only 8,000 identified as Wiccan. That means that in 10 years the Wiccan population has increased 1675%, which I think is probably pretty accurate for the movement as a whole. Projecting that at 1500%, that means that at current rate of growth, in 2010 there could be as many as 6,375,000 Pagans in the US! Now it is true that statistically, with a sample of only two points, it's impossible to tell a true growth curve. Still, if the growth rate in the next decade were even only a third of what it was in the last decade, it would make as many Pagans as Jews in the US.
Also of interest but not scientific: on ReligiousTolerance.org How many Wiccans are there?