Just curious. I have been doing mostly BB's, but am in charge of smoking some ribs for a grade school benefit, and looks like it will be St. Louis style will be donated.
Yep, like Dave said you can easily fit 8 racks of St. Louis trimmed spares, 4 on each rack. Or if you go with loin backs then you can get 12 racks in a WSM, 6 on each rack.
In either case if you needed to add another rack or two, you could lay them on top of the ribs in the rack. The important thing is to make sure you have air flow between the ribs.