How many teens are involved in the occult?

This is an impossible question to answer.

First, what are you classifying as "occult"? Serious study/practice of some tradition of magic/witchcraft? A curious trip to a palmist or tarot reader? A slumber party where the participants play with an Ouija board? All of the above? Does it matter if its just a curious investigation or a serious following?

Second, Christians/Jews/Muslims would see any form of religious practice outside their own as the practice of witchcraft or otherwise "occult". However, if you were raised in (as an example) a Native American tradition, your definition would be completely different.

Three, there are no good studies by impartial third parties of teens even just in the US, let alone globally.