Alot... The problem is its hard to get official figures for 2 reasons. The first is that most people who become Satanists dont register with any Satanic church/group/society. And secondly Anton LaVey...
At least of 400,000 in the USA.In the world, only about three million people are Witches. It seems like a big number but it really isn't if you consider that the earth holds at least at least three...