Number of bikes on regional jet

Hello,

Does anyone know how many boxed bicycles are allowed on a typical
"regional jet" in the U.S.? I recently put my boxed bike on a
Northwest-affiliated CRJ and I was told by the airline that only one
boxed bike is permitted per CRJ flight. Is this a general rule across
all airlines? If so and, say, three cyclists were flying together,
could two of them use bike bags and get their bikes on the plane too?

Thanks,
Mark