Some interesting numbers
In 2014, the Pew Research Center determined that there were 1.1 million Presbyterians in the United States, while also showing that there were 1.5 million people identifying specifically as witches, not just Wiccan.
The comparison is skewed, because the numbers of church going Presbyterians is much easier to attain, because churches keep records. Witches, on the other hand, are more difficult to nail down. The number is likely much, much higher.
-Tom Blueleaf