It's rare. I don't know who said it's common or where you get that from. Maybe fluent people say that stuttering is common but they're not really talking about the speech disorder known as stuttering or Childhood-Onset Fluency Disorder. They're talking about the disfluency that non-stutterers get when they're caught in a lie or in some kind of emotional distress like nervous or extremely frightened. This is because in all humans, the speech and emotional parts of the brain is connected. However stutterers can stutter in all situations and even when not stressed or anxious or whatever. And this is due to structural differences in the speech areas of our brains as well as the amount of white matter connecting these regions. Neurological based conditions like autism, stuttering, selective mutism (social anxiety), aphasia and apraxia tend to very rare, like 1%.