Its up to courts it seems from my reading
one website I read on if Wicca was a relgion stated One law which attempts to define a religion is: The Civil rights Act of 1964 states "To be a bona fide religious belief entitled to protection under either the First Amendment or Title VII, a belief must be sincerely held, and within the believer's own scheme of things religious." (USCA Const. Amend 1: Civil Rights Act 1964 701 et seq., 717 as amended 42 USCA 2000-16)
It also showed alot of other court rulings about it you might have interest in.
http://www.religioustolerance.org/wic_rel.htm
As for a concrete number of followers or guidelines im not sure. I know that 3 people conspiring togther is a legal gang though lol