"on the doorposts (mezzuzoth)": I understand from this (a minimum of) two mezzuzoth (i.e., one mezzuzah for two doorposts). It is, therefore, written in the second section (Ibid. 11:20) "doorposts" — "increase upon increase" (i.e., two "doorposts" followed by two), which (according to the exegetical rule) signals diminution, (Scripture hereby prescribing a mezzuzah for even one doorpost.) These are the words of R. Yishmael.
R. Akiva says: This is not needed (for the halachah). It is written (Shemoth 12:7): "And they shall take of the blood and place it on the two mezzuzoth." Let "two" not be written (i.e., it is redundant). Why is it written? To serve as a paradigm, viz.: Wherever "mezzuzoth" is written it signifies one, unless Scripture specifies two.