See the calculation with sample numbers
A 56 ft open straight run with an 8 ft maximum span needs 7 panels, 8 posts, and 14 rails when each panel uses 2 rails.
What can change the result?
This first version does not infer gates, corners, shared endpoints, property boundaries, or local code requirements.
How this was calculated: Panels, open-run posts, rails, total run length, assumptions.