Having 6 racks on at the same time may lengthen the time for the cook. Make sure they have space for the smoke/heat to get around each of them, so try not to have them touching. Perform the bend test on each rack before taking them out of the smoker. Pick up 1/4 end of a rack and if the rest of the rack bends like it is about to break or go 90 degrees you are done.
Picking it up and throwing it down on a Backyard Bomber MJH....