One at a time. It takes a few strokes to build the thicker character of the line, then complete it gradually as you get to the end of each.
Ideally, the hatching should have a clear pattern of direction that defines the form and mass of the shapes as well as the light source.
In the end, I often get the feeling that I'm simulating with my pencil the shapes I would naturally get with a brushstroke.
Hope this helps.