The right number of cameras depends on the property's layout, sightlines, entry points and your risk profile - not a rule of thumb. A cluster home in a gated estate needs different coverage to a standalone property on a large stand.
We typically plan for full perimeter coverage at gate and boundary, then cover approach routes to the house, the front door and any side or rear access. Indoor cameras are optional and depend on your preferences.
A site survey lets us map blind spots and lighting, then specify a camera count and placement that actually covers your property - rather than guessing and finding gaps after installation.