Answer:
   
   Well, the number10 denotes a complete set, because we have (assuming
   we're healthy, thank G-d) 10 fingers. Five is therefore representative
   of half of a set. The 5th letter, with a gematria of 5, is the letter
   hei. Hei denotes an outcry; that is, the letter is literally named
   "Hey!"
   
   In kabbalistic understanding of the Tetragrammaton, the letter "hei"
   represents the spreading of G-d's beneficience from a point outward.
   It it therefore composed of a point-like yud and a dalet showing
   orthogonal axis, 4 (the gematria of dalet) compass points. We find in
   Genesis 1 that creation can be described through the metaphor of
   speech. "And G-d said 'Let there be light!'" So, this permeation of
   G-d's Goodness through the universe is very much an outcry. The Talmud
   sees in the shape of the letter the theme of repentence -- the choice
   of descending or finding that small window near the top. They too
   touch on a theme related to outcry -- but not G-d's call to man, but
   man's cry to G-d.
   
   The song toward the end of the seder asks "Who knows one?" and makes
   its way up to 13. For 5, the answer is "5 are the books of the Torah".
   Which is why there are 5 books of the Torah -- because only with
   including the Oral Torah with the written text are we dealing with a
   complete set. This idea, of two halves crying out for each other, is
   what the symbology of five revolves around in Judaism.