From your service back to the transformer through earth would be one resistor in series no matter how many rocks you magnetic will pick up.
It is not a matter of how much voltage is present it is a matter of how much current will travel through this resistor (earth).
Check the resistance of your rod using the three point resistance test for ground rods but the equipment is very expensive. You could use a clamp-on resistance tester but they arenít very accurate.
This will tell you the amount of current that will travel throung earth at your house.