This is more of a further question than an answer. I'm assuming you're doing 2 shifts at least on this, as I can't see 8 whole breasts fitting at once in the WSM, unless they're small-- but you've got 20 to feed. If you butterfly, I could see one on top and one on the middle. Not butterflied, I could see 2 and 2, with some attention paid to alternating the sides exposed to the higher heat coming around the edges of the grates. If you cook at 325 and leave them whole, you could probably do 4 and then 4 more in about 5-6 hours total. Lower, and you might need 3 hours cook time per shift, plus whatever time it takes to get the coals fired up.
The ones pictured on this
page are small and boneless. The last time I did turkey breast, it was a half, bone-in and nearly 4 lbs by itself. See
here for reference. You might need to take the tape measure to the store.
Like I said, I didn't really give you an answer, but maybe some food for thought.
Umm, by the way, you are using a WSM, aren't you? /infopop/emoticons/icon_rolleyes.gif