How much heat do 120 volts of electricity generate in 12 gauge copper wire?

The volts won't generate any heat at all. It's the amps (current) that generates the heat. The 12 AWG wire is designed with different types of insulation that can handle temperatures expected in various conditions.

If a 12 AWG wire (or any wire) is overloaded, then it becomes more like a resistor than a conductor, generating heat, increasing the resistance, increasing the current, in a thermal runaway until there is enough heat to melt the wire (assuming the overcurrent protection has not already disconnected the power).