How much voltage before Zener diode will be physically damaged?
If you do this, it will go &quot;bang!&quot;:



simulate this circuit &ndash; Schematic created using CircuitLab
The Zener diode will act like a short circuit, and try to pass all the current the supply can deliver.
If you do this, it will be fine:



simulate this circuit
The difference between &quot;bang!&quot; and OK is that resistor.  It limits the current through the Zener diode and allows it to safely regulate the voltage.
In the circuit as shown, the current through the resistor is \$I = \frac {E}{R} = \frac {V_{Supply}-V_{Zener}}{R1} = \frac {20V - 12V}{1000 ohm} = \frac {8V}{1000 ohm} = 8mA\$.
Since the resistor and the Zener diode are in series, the current through the diode is the same as the current through the resistor.
As long as the Zener current is below the maximum rated current and the product of the current and the Zener voltage is less than the power rating for the diode, then the diode will be fine.