Joe Lee's already decided to go with the big boy. Understand, but just for support of the little ole 18", it can hold a bit.
I use Sams Club three pack of spareribs. They are too long for the 18" circle of the top rack when you put two on, so I trim one end (about 3-4 inches) from all three and load it up. So there's one rack of ribs in the center and the trimmed end pieces on the sides of the bottom rack and two racks of ribs on the top rack, all laying flat. I don't trim the width down St. Louis style, so the WSM racks look jammed with the raw meat, but I still get great results.
Those trimmed pieces cook faster on the outside of the bottom rack, so you can pull them a little early for appetizers.
I've only cooked for a maximum of 8, so this is plenty. Going for bigger crowds, you would definitely need to use ribholders to fit more.
The one brisket I've done just fit, so luck involve there.