USA Today had a piece recently that looked at the unusual fact that there were four mass killings in the US in the span of four days. These killings took the lives of nineteen people.
"Murders don't distribute themselves evenly over a 12-month period," Levin said. "Just because we see four occurring in proximity to each other doesn't mean we are suffering through an epidemic of mass murders but that's what people will think."
Via USA Today
I'm not sure that there is ever a "normal" number of mass killings since they are such extraordinary events.