The last four words of Maoz Tzur end with the words הקם לנו רועים שבעה. This is a clear reference to the verse (Micha 5:4):
וַהֲקֵמֹנוּ עָלָיו שִׁבְעָה רֹעִים וּשְׁמֹנָה נְסִיכֵי אָדָם
But as I was singing it yesterday it occurred to me, why did the composer choose the number seven? Wouldn't it be more appropriate if it said נסיכי שמונה referencing the end of that verse and corresponding to the eight days of Chanuka? You would get the same effect (i.e. the hope for redemption and the Messiah) and poetically and symbolically it would look more pleasing this way. Why did the composer choose the number seven, and what is it supposed to signify?