It's hard to tell sometimes. We've had as few as 9 cars, and as many as 40. These usually get posted on a number of forums, so it's hard to even get a feel for general interest.
If you're concerned about traveling too far for a poor turnout shoot steve(sd203) an email. He's the one who coordinates everything, so he can give you an idea of the response he's been getting. firstname.lastname@example.org
You can also find the original thread at http://forums.bimmerforums.com/forum....php?t=1185591