1-His brother, John, is coming over for a visit.
2-His brother John is coming over for a visit.
Does 1 necessarily imply that he has only one brother?
Does 2 necesssarilly imply that he has more than one brother?
They imply that, yes. However, don't be surprised if people use the commas either way with either meaning.
His [one] brother, [who, by the way is named] John,
His brother John [as opposed to his brothers Paul, George, or Ringo] is..
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.