According to Deuteronomy 17:14-17, there are limits to a king's possessions:
When you have come into the land that the Lord your God is giving you, and have taken possession of it and settled in it, and you say, "I will set a king over me, like all the nations that are around me," you may indeed set over you a king whom the Lord your God will choose. One of your own community you may set as king over you; you are not permitted to put a foreigner over you, who is not of your own community. Even so, he must not acquire many horses for himself, or return the people to Egypt in order to acquire more horses, since the Lord has said to you, "You must never return that way again." And he must not acquire many wives for himself, or else his heart will turn away; also silver and gold he must not acquire in great quantity for himself. [emphasis added]
For when Solomon was old, his wives turned away his heart after other gods; and his heart was not true to the Lord his God, as was the heart of his father David.
This verse seems to imply that 700 is too many, but seven is not. Is that a fair reading? Is there a way to narrow the gap?