I don't know if it will be accurate, but I've seen predictions of 5,000 Muslims and 50,000 bikers. We'll see.
I'll bet there won't be more than a couple hundred Muslims. This is a fringe group and the other muslim organizations like CAIR and ISNA are distancing themselves as much as they can, not even mentioning it on their sites . Interestingly, though, a Pennsylvania Tea Party group has allied itself with the muslims and are supposed to show.