The number of concurrent users is limited by the operator’s ability to reuse frequencies across geographies, which in turn is limited by the angular resolution

The number of concurrent users is limited by the operator’s ability to reuse frequencies across geographies, which in turn is limited by the angular resolution of satellite antennas and satellite altitude (ceteris paribus). Inmarsat is mostly marketing to ships at sea, and other cases where users are quite far from each other. So they can use a small number of geostationary satellites. Starlink is marketing to “regular people” in exurban and rural places that are uneconomic to serve with DSL or cable. So they need to create very small cells, hence low altitude satellites. But low altitude satellites can’t see as much of the ground at once, so they need a lot of them. Also, the geostationary model doesn’t provide good coverage of high latitudes (and launching geosynchronous satellites in high inclination orbits to cover those high latitudes would multiply the number of satellites needed by Inmarsat). As a result, there are basically two models for providing satellite comms: a few geostationary or a bunch of polar LEO.