Camera count is not coverage. A site with fifty cameras and no coverage plan will still have blind spots — and a site with half as many, placed against real detection and identification distances, will often perform better.
Coverage planning starts with defining what each camera needs to achieve: detection (something is there), recognition (what kind of object or event), or identification (specific enough for evidential use, typically a face or a plate). Each of these requires a different pixel density at the target distance — identification needs roughly four times the pixel density of simple detection.
From there, coverage is a function of lens focal length, sensor resolution, mounting height and distance to the area of interest — not a fixed number of metres “covered” per camera. A wide-angle camera covering a large area at low pixel density may satisfy a detection requirement but fail completely for identification.
The other variable people underestimate is bandwidth and storage: resolution and frame rate decisions made camera-by-camera add up fast across a large estate, and retention periods driven by insurance or regulatory requirements can dominate total system cost more than the cameras themselves.