The Plait Count is how many strands were used to make the overlay (the part on the surface you can see).
12 Plait uses less strands, and is lighter. Good for faster routines.
16 Plait uses more strands, and is thicker and a little heavier, but has a smoother rollout as a result.
Most whips have at least 2 bellies (braided layers underneath that serve as the whips muscles). A whip with 3 bellies is larger and heavier, but has the best rollout.