I'm only beginning at drawing lineart digitally, so I use different layers. I do the general shape in one layer, then I do the details in different layers, but when I'm happy with the result, I merge all the lineart layers into one.
Depends on how many mistakes you make. I often do detailed work in a different layer because, when I make a mistake then, I don't identically erase the base lines that were good. But it's all a matter of personal preference.
I used to use a whole bunch of layers but now I just stick to one since it was too much to jump between layers all the time. If it's something I know will take me a few tries before I like it, then I'll use another layer.
for painting and airbrushing I use only one layer since it doesn't matter at the end it'll be erased, but when it comes to anime, I usually do the body in one layer, the hair on another and the background on a third one.
I do it pretty much all on one layer. The only thing I'll put on a separate layer is stuff like hair or fur, because that changes so often when I paint it that I want to be able to easily adjust it without screwing up the rest of my lines.