On the 18 you can probably do 3 on the top rack if they're not too big and 2 on the bottom. On the 22 , you can easily do 4 average size on the top and 3 on the bottom. Anything more than that you might have some air circulation problems.
On my 18, I've done 3 picnics on both the top and bottom together. It was about 52 lbs total weight. I stood them up like a tee pee with a little foil roll at the top where the bones came together.