Dragon 1 appears to have them arranged in 2 sets of 4, and 2 sets of 5, as seen in this photo:
Yet Dragon 2's layout is a bit different, if everything is symmetrical, it appears to have 4 sets of 3, and the 4 lone thrusters on the inside of the SuperDraco mold lines:
This produces a sum total of 16 Dracos. So, what's right? Additionally, if there is a difference, why?