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