You could make an estimate of adults and if they actual go or not,
would it matter? I would suspect you would be better having declared a
few extra than none at all... Just my opinion after looking at the
permit and such at the BSA website. An attorney or the BSA leadership
should be able to advise more specifically, though.

That's probably a good idea. Technically, BSA wants releases before you
file the permit. In my experience the boys that show up and not
necessarily the boys that signed up. I generally take a best-guess
attempt and just have unexpected boys (parents) sign the release when
they get there.

-Robert