שמואל ב, פרק ה׳, פסוק י״ג

As David's reign in Jerusalem takes root and he realizes that God has elevated him to a great kingship, he begins to expand his royal household. This expansion takes place only after his transition from Hebron, signaling a time when the kingdom is finally stable and secure [ביאור שטיינזלץ, חומת אנך].

In expanding his family, David takes concubines before taking additional wives, a specific sequence that highlights his newly elevated status. For an ordinary person, taking a concubine is forbidden; it is a unique privilege granted exclusively to a king [חומת אנך]. The exact difference between a wife and a concubine is a matter of discussion. The primary approach among commentators is that a wife is taken through formal betrothal and receives a marriage contract, whereas a concubine is betrothed but does not receive a contract [מלבי״ם, אברבנאל, רד״ק]. Because concubines still undergo a formal betrothal, the parallel historical account in the Book of Chronicles groups them together with the wives under a single category [מלבי״ם, אברבנאל]. However, another perspective suggests that a concubine is taken without any formal betrothal or marriage contract at all [רד״ק].

In total, tradition records that David had eighteen wives and concubines. Six of these wives were already married to him during his time in Hebron. Once he moved to Jerusalem, he added twelve more women to his household [רד״ק, אברבנאל]. Out of these twelve, only two were full wives, one of whom was Bathsheba. The remaining ten were concubines, who are later remembered as the women David left behind to guard his home when he fled from his son Absalom [רד״ק].

Through these unions, more sons and daughters are born to David. The parallel historical record in the Book of Chronicles includes two additional sons, Eliphelet and Nogah. They are absent from the primary list of his children because they passed away while they were still young [מלבי״ם].