You don't need Ohms law dummies book any more, Google can be your friend, or even Alexa, no?
In fact, you don't even need Ohm's law. As TLS Guy mentioned, the power formula for a
resistive load are:
Power = V^2/R, or VI, or I^2*R so if using P = I^2*R, I = √(P/R).
Example: If the amp is rated 100 WPC into 8 ohm, rated current output would be:
I = √100/8 = 3.54 A per channel
Typically, a good mid range AVR can output at least 15 to 20% more power into 4 ohm, so assuming 20% more power into 4 ohm, the current output into 4 ohm would be:
I = √120/4 = 5.48 A per channel
The above 4 ohm load examples are for 1 or 2 channel driven only. If 5 or more channel are driven simultaneously, most mid range AVR, and even some flag ship models will have trouble delivering the much higher current, and would likely shutdown by the protection circuit.