Spatchcocking is just removing the backbone and laying the whole turkey flat, so only need two racks. Here’s a video from a site I reference and visit often when it comes to technique and recipes. The author of this article has a book called “The Food Lab” that is also great!
https://www.seriouseats.com/butterfiled-roast-turkey-with-gravy-recipe
I have not cooked whole chicken or turkey in the Smokin-it. Only turkey pieces like legs and bone-in breasts. And the whole chicken’s I’ve recently done have been on my Weber Kettle, spatchcocked as well.
But, either way, go with what you think is going to give you what you are hoping for.