In Hugin, should control points for a panorama made from exposure stacks be set on every exposure or just one per stack?

Usually you only need control points on one image from each exposure stack, typically using the same exposure level across the panorama, as long as the images within each stack are already well aligned.

If the brackets were shot on a tripod and there was no movement between exposures, Hugin can often use its stack-detection/linking so all images in each stack share the same position. In that case, control points for one member of each stack are generally enough.

If the stacks were shot handheld, or there may be movement between exposures, align each stack first before doing the panorama alignment. Otherwise the stacked images may not match properly.

Also, you may not need to place control points manually at all—Hugin’s automatic alignment on the Assistant tab will often create the needed control points for you.