You
might get eight if you hang them on this
Weber 7472 Expandable Smoking Rack, 18-Inch
but I think their weight will probably be more than that rack can handle. It's meant for fish and sausages. Also, the racks may be too long once hung (the inner 2 grills are removed but the bowl remains in place).
Like Phil says, rolling is the only solution I can see (and then only 6--3 on top & 3 on the bottom).
You might squeeze 4 on each rack but only if they're smallish. Keep in mind if the meat touches other meat, no bark will develop on those areas that touch.
CONSIDER THIS: You might try an experiment that I haven't seen anyone try: stack two.
Coupled with rolling six, roll the 7th and 8th and put one on top of each 3-pack. The bottom "4-pack" may not fit height wise but you can "lift" the top rack so the bottom can fit by putting 3-4 empty tall boys on the bottom rack so the top rack is resting on the 4 cans instead of the barrel tabs. There are more permanent solutions to lift the top grate but this is a quick and cheap temporary solution. PLUS it's fun emptying the needed beer cans
You won't get bark where the ribs touch but it should work in principle (all the racks WILL be cooked as desired). I have NOT tried this, it's just an idea, depending on how desperate you are for 8 instead of 6 racks.
If you try this, be sure to post pics and let us know how it turned out.