What does the Torah consider too many wives?
Rashi and Targum Yonasan: More than eighteen (like we find by Daved ha'Melech). 1
Rashi: Who had six wives when the Navi - in Shmuel 2, 12:8 - said to him "ve'Im Me'at, ve'Osifah l'cha ka'Heinah ve'cha'Heinah" (I will increase for you like these (another six) and like these (another six). Rashi (in Melachim 1, 1:4): And it was because he already had eighteen wives that he did not allow himself to be intimate with Avishag ha'Shunamis. See also Torah Temimah, note 87, s to why the Gemara does not Darshen that he is Chayav for each wife, like it did by horses in the previous Pasuk. Refer to 17:16:2.3:1.
Why does the Torah insert the (otherwise superfluous) word "ve'Lo Yarbeh lo Nashim"?
Sanhedrin, 21b: To extrapolate that a Hedyot is permitted to have as many wives as he wishes. 1
See Torah Temimah, note 86.
Why does the Torah insert the (otherwise superfluous) word "ve'Kesef ve'Zahav Lo Yarbeh lo Me'od"?
Why may a king not have too much silver and gold?
Targum Yonasan: So that his wealth should not go to his head and cause him to become vain and to rebel against Hashem. XX
Ibn Ezra: To prevent him from imposing excessive taxes on the people.
Alshich: To prevent him from turning his heart away from Hashem in order to pursue his desires. 1
Refer also to 17:17:152:1.
The three current prohibitions correspond to the three things that take a person out of the world 2 (Avos 4:21): Jealousy - too much money; Desire - too many wives, and Kavod - too many horses (See the first Pasuk in Shiras ha'Yam).
Why, with the exception of the two current Mitzvos "Lo Yarbeh lo Susim" and "ve'Lo Yarbeh lo Nashim" does the torah not give reasons for the Mitzvos?
Sanhedrin, 21b: Because the Torah gve the reasons for these two Mitzvos, 1 and see the result - Sh'lomoh ha'Meleach declared that he would marry more wives and they would not turn him away, and he would acquire more horse and would not go back to Egypt - and ultimtely he transgressed on both issues. 2
Why did the Torah not give the reason why he may not acquire excessive silver and gold, like it gave for increasing horses [and women]?
R. Bachye: The Torah does give the reason - when it writes in Pasuk 20 "Levilti Rum Levavo ... " (and "Levilti Sur min ha'Mitzvah ... " - Da'as Zekenim, Hadar Zekenim and Rosh [all in Pasuk 20]). 1
Alshich: "ve'Lo Yasur Levavo" written in connection with not increasing wives applies also there.
Da'as Zekenim, Hadar Zekenim and Rosh: Which is also the reason why he must have an extra Seifer Torah - See Pasuk 19.
QUESTIONS ON RASHI
Rashi writes that a king may have up to eighteen wives, based on the Pasuk "ve'Osifah l'cha ka'Heinah vecha'Heinah" (another six and another six). But "ka'Heinah" should add six, and "ve'cha'He'nah" twelve more (like he will have after adding ka'Heinah)?
Moshav Zekenim: "Ve'cha'Henah" like "ka'Heinah' means 'another six'.
