Yes, World has several really nice illustrations of various time spirals (one of which is the Mystic sand painting we see in the beginning of the movie), with the urSkek symbol art around the crystal being the master template. Both the illustration and the text support the idea of 18 urSkeks, with the overhead view of the symbols being in groups (two each of two, three, and four). I wouldn't be surprised if perhaps this number doesn't necessarily line up with the onscreen props, as Jim said he liked the idea that the Skeksis embody the deadly sins even though they number more than seven. He couldn't recall if they doubled some sins or invented new ones. I stand corrected, because my brain is not good at math today.