How to determine max amps over a common ground between AC-to-DC Adapters?
Since the common ground connection should be only acting as a reference equalization for the control signal, there should be very little current (a few mA) running through it.  If you are concerned, hook up an ammeter and verify it as you add boards.  I'd be more concerned about trying to run 40A through the traces of the first board connected to the power supply.