How many boxers have gotten a dent in their heads ?

It's really very hard to land a full impact punch vs a boxer. The body simply reduces impact, if the impact is powerful enough to shut down the CNS, the legs will be gone and no extra damage can be done. It's very unlikely that any human punch can break a skull, otherwise we would have that.

In assaults, it's usually a defense-less human receiving repeated hits in the same spot, usually by some kind of object that delivers similar or more PSI than a KO punch. The KO has minimal physical damage (dent) and leaves a real psychological dent, if the boxer doesn't fully understand what the KO is. A boxer who takes a 12 round Low PSI pummeling will suffer more physically than a boxer who gets KOed by the first punch.