||He [also] must not have many wives, so that they not make his heart go astray. He shall likewise not accumulate very much silver and gold.
Velo yarbeh-lo nashim velo yasur levavo vechesef vezahav lo yarbeh-lo me'od.
Although polygamy was permitted, and was common for kings, an Israelite king was forbidden to have more than eighteen wives (Sanhedrin 21b; Yad, Melakhim 3:2).
That is, the king is forbidden to build up a personal fortune (Yad, Melakhim 3:4).