The non-volatile shutter count is stored in a camera chip. One way it is reset is when replacing that board with that chip. Possibly there are other ways, possibly when the shutter is replaced, but I think it is not revealed to us. The shutter count is NOT limited to 64K, shutter counts of 200,000 or 300,000 are not uncommon. 24 bits will store a count of 16+ million.