[AJerman]

It really goes either way depending. As said, it depends on if you're refering to individual hairs, or all of your hair in general, or in this case, each individual hair on the back of your neck, or all of your hair on the back of your neck.

However, I will give you that no one ever says "It made the hairs on the back of my neck stand up". It's always "It made the hair on the back of my neck stand up". However, upon further inspection, it COULD go either way.