From what I understand, The ESC is rated to withstand a maximum current (amps). If the motor and servos draws a current of, lets say, 20 amps, than you'd be best to get an esc rated 25 or 30 amps. This way, you have a buffer, just in case there is any unforeseen spike in current draw.

Hypothetically, a 100 amp ESC is capable of running a electric setup that draws 1 amp up to 100 amps.

Please correct me if I am wrong folks!

~Noah
***Edit*** Words in bold replaced words to better suit what I was saying