Depends. Lots of factors.
First thing I would ask is when do you fish for crappie? What time of the year do you want to catch fish over them?
You can build shallow stuff (to me that is less than 6' deep), you can put them out in deep water (20-30' deep) and fish them in winter and summer. You can have the only structure on a barren flat and hold every fish or you can put them along natural travel routes like ditches and creek channel edges and ledges to hold fish.