Brian,
The shot class shelves are ~9 studs deep. I used 4x8 plates on the bottom and built up the displays from there. Each step/tier is two plates higher than the one before. The number of plates needed per shelf would vary based on your design and type/size of plates used.
Hope that answers yout question.
B
Quoting Brian Pauls
I was wondering how many black plates I would need to get to make the 3 tier displays on each shelf. I collect star wars minifigures and your display is one of the best I've seen.
Thanks, Brian