Only in a classic British state-owned company could so many callsigns be in simultaneous use - in what was (as far as the travelling public knew) a single airline business.
This is nothing compared to the major US airline hubs nowadays, where the dominant carrier will have every Tom, Dick and Harry of a subcontractor running "commuter" flights under their own callsign, while in the full livery and branding image of the mainstream airline.
That's until they have an accident, when the main carrier instantly distances themselves from the operator.