Bearing in mind that "Mezuzos" is plural, does this mean that why do we affix the Mezuzah only on one doorpost?
Rashi: Because "Mezuzos" is written without the second 'Vav' 1 , so it can be read 'Mezuzas' (singular).
R. Yishmael (Menachos 34a): Because it says twice Mezuzos (plural). Ribuy after Ribuy comes to exclude.
R. Akiva (Menachos 34a): We learn from "v'Al Shtei ha'Mezuzos" (Shemos 12:23, regarding Dam Pesach) that everywhere else, Mezuzos is one.
Whereabouts on the doorpost does one fix it?
Targum Yonasan: On the top third of the doorpost. 1
Peirush Yonasan: See also Na'ar Yonasan. Menachos, 32b: On the Tefach that is closest to the street.
To which doorpost does one affix the Mezuzah?
Targum Yonasan: On the right doorpost (from the perspective of outside the house). as implied by the word "Beisecha."
Menachos, 34a: "Beisecha"
What are the implications of the word "Beisecha"?
Targum Yonasan and Menachos, 34a: Refer to 6:9:1:5:1 and note.
Rashi (in Bava Metzi'a, 101a): "Beisecha"
What are the implications of the word "u'vi'Sh'arecha"?
Rashi: It comes to incorporate the gates of one's courtyard and of the Medinos and of the towns in the Mitzvah of Mezuzah. 1
Ramban: As in 15:7.
Rashi writes that "Mezuzos" is written without the second 'Vov'. In our tradition, it is full!