What amount shocks does it usually take for defibrillator to work?

240-300 J: 240 to 300 joules (depending on the machine) is usual.

Usually One: The goal with defibrillator therapy is to minimize shocks. Most internal defibrillators are programmed with a high enough energy that they will often but not always be effective on the first shock. There are many variables to consider however and the data for external defibrillators (AED's) is different.