I think the idea of the "seven couple set", or "borrowing", works for any
number of couples that is of the form 4 + multiple of 3, i.e., for 7, 10,
13, ... couples, but is practical only for the numbers mentioned. Even
there, people usually seem to prefer to arrange 10 couples as two 5 couple
sets, and 13 couples as two 4s and a 5. I don't think the idea works for
9 couples. Also, it only applies to 3 couple dances - although I have seen
people trying to apply it to 2 couple dances!