Rivian hasn't announced how much weight the roof rails can support. We only know that the R1S has 7.1 cu ft of rear storage and 11.1 cu ft of front trunk storage. It also has a towing capacity of 7,700 lbs.
If Rivian gets it right, this might be the ultimate off-the-grid vehicleFWIW, there have been plenty of pictures and videos showing a tent erected across the rails attached to the bed walls of the R1T. Such pictures/videos have implied at least two combinations of tent occupants: 1 male, 1 female, and a recent video of two female Rivian employees went on a camping road trip with R1T.
So factoring in the weight of the tent, 2 persons, some gear (sleeping bags, luggage, lantern, etc) you might be able to in the mean time come up with a rough minimum weight the rails could handle.