What is the single most important factor determining the required quantity of satellites for coverage?

The height of the orbit

The single most decisive factor that dictates the quantity of satellites required to blanket the globe is the altitude at which those satellites are placed within their designated orbit. Satellites positioned very high up, such as those in Geostationary Orbit (GEO), maintain a fixed position relative to the equator and can observe a massive geographical slice of the planet, thus demanding the lowest total count. Conversely, satellites orbiting much closer to the surface in Low Earth Orbit (LEO) possess a significantly smaller field of view relative to the ground. This reduced line of sight necessitates a far greater number of spacecraft to ensure that the coverage footprints interlock sufficiently to avoid gaps, making orbital height the primary determinant of constellation size.