The number of satellites required for 100% coverage of any network is a function of a number of variables such as orbit radius, capacity of each satellite, footprint of their beams, etc. The number required for GPS, for example, is rather different to the number required for the Iridium constellation or Starlink.

You may wish to look into the Walker Constellation topology if you really want some justification.

The number of satellites required for 100% coverage of any network is a function of a number of variables such as orbit radius, capacity of each satellite, footprint of their beams, etc. The number required for GPS, for example, is rather different to the number required for the Iridium constellation or Starlink.

However, GPS, GLONASS and Galileo all use approximately 24 satellites so you could make the case that for a positioning network based around TDOA this is the de-facto norm. You may wish to look into the Walker Constellation topology if you want more detail on this popular design.