A geostationary orbit will keep the satellite in one area relative to the surface

No. Geostationary orbits are equatorial, but equatorial orbits are not necessarily geostationary. To be geostationary, the orbit needs to be equatorial, circular and at the altitude such that one orbit takes one sidereal day (approximately 24 hours 3 minutes 56 seconds. ) An equatorial orbit need only be located above the equator, may have any period and need not be circular.

The absolute minimum number is for momentary complete cover is 4 in a triangular prism shape, but these will not stay in the right positions as they are not in a geostationary orbit.

3 in geostationary orbit will give very bad and failing coverage at the poles and at the outer edge of their footprint

6, 3 in polar orbit and 3 in geostationary orbit will help, but for a practical coverage 4 in geostationary orbit and 4 in polar orbit will be more practical

Geostationary Earth Orbit