13.
Honestly quite mid, but it is unique. There are not a lot of snake mons out there when you think about it. We got Arbok, Seviper, and Serperior. If we stretch it to serpent-like mons then Milotic, Huntail, Gorebyss, Orthworm, and Eelektross would count. That's not even counting the 2 centipede mons, Scolipede and Centiskorch.
Bottom line, I want more snake mons. Specifically snake and not just serpent-like.