Question: Why are ancient Jewish Kings permitted to marry up to 18 wives and not more?

Answer: Thank you for your excellent question! In the Torah (Deuteronomy 17:17), it says, “And he shall not take many wives for himself, in order that his heart will not turn away”. If a King has too many wives, he may become ego-maniacal and forget about G-d. Also, if the wives are from foreign religions, although they had to convert before he could marry them, he might not be able to determine whether their conversions were sincere, since it’s hard to keep track of so many wives, and may thereby bring idolatry into his palace.

So we see that too many wives for a king is forbidden by the Torah, but how do we derive how many is too many? The classic commentary of Rashi on that verse in Deuteronomy says the limit of 18 wives is from the Talmud, where it is derived from another verse in the Bible, II Samuel 12:8, where the prophet Nathan tells King David, who had six wives at the time, that “if this is too little, I would add for you like them, and like them.” Each time it says “like them” means to add the same number, so since there were six already, Nathan would say it would be OK to add that many twice, which would total eighteen. If that number of wives were not permitted for a king, the prophet would not have suggested it.

All the best,
Rabbi Kolakowski