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Hmm.
Connecting multiple slots shows it returns "number of explicit connect calls+1".
Running
obj.destroyed.connect(print)
obj.destroyed.connect(print)
obj.destroyed.connect(print)
then causes obj.receivers(signal) to return 4 (number of explicit connects + 1).
So I assume it counts some sentinel value or internal cleanup slot that is only placed/connected if anything is connected explicitly in user code.