I have always had a problem understanding electricity. I have what to some may be a simple question.

Suppose an AC circuit with 120 volts and 20 amps. If I plug an extension cord into that circuit and assuming it's of sufficient size to adequately carry the load, how many other cords can I plug in to that line? How do multiple cords affect the amperage? voltage? Is there a formula for this? And I guess the length of the cords comes into play here also.

Now if someone can answer that in easy to understand terms, I would appreciate it very much.

Thank you.