Seeing as Dan had only one son called Shucham, why does the Torah write "Mishp'chos Dan" in the plural?
Ramban: Because Shucham had many children, each of whom formed small families which all combined under the banner of Shucham. 1
See Ramban.
What did Shucham (was was a deaf-mute) do to merit so many offspring?
Oznayim la'Torah: Perhaps it is because he killed Eisav, when the latter delayed the burial of his brother Ya'akov. 1
See Oznayim la'Torah.
Why does the Torah conclude "Kol Mishpachos Dan" - which it does not do by any of the other tribes?
Oznayim la'Torah: To stress that, in spite of the vast number of offspring, they were all from his one son Chushim.