Several references indicate that average person poops between 3 times per day, to once every 3 days.
Any group could average anywhere in that range.
Given the unpleasant prospect of re-using a wag bag, I suggest using the higher range of (3 per day) and adding a couple extra just in case.
But I am not sure this is group question. I think it is an individual question. I would assume each person would carry their own wag bag supply, in and out of the area. Thus each person would make their own decision based on their personal experience and desire to both meet the requirement and not have to use the same bag multiple times.