You won't be able to fit as many ribs in this way, but I like to roll my ribs. I can get three racks of ribs on each grate (so six on my 18.5" wsm). The main reason I do this is the same concern I will alert you to as you look for a rack. If you really want to clean your rack between cooks, most rib racks are a pain in the butt. I have a rack that can hold a few more ribs, but it's not worth it. Cleaning the rack takes too much time. With rolling, I skewer the meat and when done, the only thing I have to clean is the wsm grate, which I would have to do anyway. The skewers are bamboo and I obviously just throw them away. There are racks that are easier to clean than others so keep this in mind as you consider options. Basically, the hard place to clean is where metal comes together on the rack so the less of that there is, the easier it is to clean. You also want the gaps to be big enough to get your hand in there to clean the rack.
Another option is to add another grate or two. There are plenty of ideas in the mods section, but basically, just stack another grate between your top and bottom grate and you've increased your capacity. And in my opinion, a grate is much easier to clean than a rib rack.