generally i'd say 3 pairs of panels, with the rod below the arch. but for some reason the proportions seem off and i think your space would look squat.... so i'd do the rod at (close to) ceiling height, one pair for the whole width. the amount of stack back would depend on whether you want to close them ever or just frame the windows. in any case, the rod should be wide enough for the amount of fabric you choose to be totally off the window, just leave the outer casings covered.

like this.... i'd skip the middle set of panels.