Subtracting 4 about inches for space between the rf plate and first rack and about an inch for the rack leaves 19 inches of free space, second rack will reduce that to a bit less than 18” of “extra” space to work with. Additional racks are now a function of a realistic assessment on what you plan to cook and how much. If you are planning to occasionally smoke large critter parts, standing rib roasts, whole turkeys, etc you will need a lot more space than someone who only ever cooks butts, briskets, ribs and chicken parts. Whichever spacing configuration you chose needs to provide about 2” of free air space above whatever is loaded on the rack. Also keep in mind each additional rack will have a effect on air flow and temps within the cc.
Personally, on a 24x36 build, I use two racks, with around 9-10” clearance over the top of the bottom rack.
Good luck.