How would electric field lines look like if charge densities on parallel plates (like the ones used in parallel plate capacitors) are different (still opposite though so electric fields can form)?
Electric field lines are a pictorial tool to provide general qualitative information on the nature of the field, specifically the direction and relative strength of the field. The direction is indicated by arrows on the lines. The relative strength in a particular area is indicated by comparing the density of the lines in that area to other areas on the same diagram. The density of the lines provides no quantitative information on the field strength.
Since the strength of the field between the plates of the capacitor is proportional to the charge density, increasing or decreasing the charge density, all other things being equal, would increase or decrease the density of the lines, respectively. However, the density of the lines of a diagram of a single capacitor will tell you nothing because there is nothing to compare it with, such as another capacitor on the same diagram which is identical except for the charge density.
Hope this helps.
As always, the elelectric field lines begin on positive charges, and end on negative charges. Even more precisely: According to Gauss's law the net number of field lines (emerging lines minus entering lines) from a volume element is proportional to the charge enclosed in this volume.
Here is an example with two parallel plates. The positive charge density on the left plate is around 3 times the negative charge density on the right plate.
Therefore the number of field lines beginning on the left plate (22) is around 3 times the number of field lines ending on the right plate (7).