There is no concrete number. After a number of defibrillations, however, your heart can undergo electroporation where the threshold power required to defibrillate will become much higher--and then...
For EP, try "Electrophysiologic Testing" by (I think) Dan Fogoros. It's the best, brief, down-and-dirty EP read out there. It's a little old, but most of the info is quite relevant. Be advised that...