How often does the electric shock revive someone who had a cardiac arrest?

Reset conduction. You heart has an electrical conduction system that controls how the heart muscle contracts. Often in cardiac arrest that conduction system is sending disordered electrical signals. The shock sometimes given during cpr/acls is an attempt to "reset" the conduction system so the electric signals go back to the proper order.
Depends. This depends on lots of factors, whether there is a heart rhythm that can be "shocked" , whether the arrest was witnessed and whether or not the patient recieved cpr. Finally the cause of the arrest matters. In the best case, someone sees the arrest, the rhythm is one in which a shock will work, and is done quicky, about 70% will regain a pulse, with 40% surviving to leave the hospital.