Each of the four pieces uses 4 slots (plus some overhang on each side) for a total of 16 slots if you use all four pieces together.
16-slots cannot be right as all the pieces are not the same size. also, all 4 pieces of the XTM Hand Stop put together is 4.7" long. A 13-slot 1913 Pic rail is 5.3" long, so you would probably need an 11-slot or 12-slot rail. I am also trying to confirm the EXACT number of slots needed, but it is NOT 16-slots for sure.
The overall length of all four panels is about 5.5" as shown in minute 2:25 of this review on YouTube: https://www.youtube.com/watch?v=zQ7KLpE2AfI. If you look at minute 1:40 of this other video (https://www.youtube.com/watch?v=TyzBbZBck0Q) you can see that the hand stop overhangs slot B6 on the back end and slot B20 on the front end. Hopefully that helps.