You don't need a cluster of HackRFs to see a single beacon. But the combination of several HackRFs does give you larger instantaneous bandwidth and the ability

You don't need a cluster of HackRFs to see a single beacon. But the combination of several HackRFs does give you larger instantaneous bandwidth and the ability to track an individual beacon over a wider range of Doppler shifts and hence for a longer time / part part of it's orbit. And the ability to see a whole bunch of beacons at once.