No. Of Railing Runs:

Up to 4 railing runs can be entered. Each railing run can have a different length of run and height.

For example a run maybe along the pavement side of your house and then two further runs separate your garden from your left and right side neighbours. Different considerations may determine the height, eg perhaps one railing run is to be fitted ontop of a small wall which makes its height shorter so that it matches the height of the other railing runs.