There are a lot of field lines, so it’s useful to be able to draw simpler patterns still, selecting a typical field line. There is no rule for the correct number of field lines—just draw as many as are useful to think with.
Building simple representations to capture the essence of a situation is never easy. Success is judged against a purpose, and being able to predict a change is often a touchstone for physics. You’ve not yet looked at how field patterns change, so for now typical lines should capture how much the other lines are distorted by the radial forces of other field lines and axial force of their own tension: that is how much they could straighten or shorten when the lines are allowed to relax.