What does the number or density of field lines on the source charge indicate

The strength of the field

Explanation:

The density of field lines of a field (e.g. gravitational field, or electric field) indicates the strength or magnitude of the field itself. In particular:

- The regions in which the field lines are closer to each other (more dense) are the regions where the field is stronger

- The regions where the field lines are more spread apart (less dense) are the regions where the field is weaker

For example, if we consider the electric field generated by a positive point charge, the electric field is radial (outwards from the charge) and the spacing between the lines increases as we move away from the charge: this means that the strength of the field increases as we move away from the charge.

the doppler effect is used by weather forecasters to measure 

moving clouds