Midrasch zu Schemuel II 13:13

He (the king) must not marry more than eighteen wives. Whence is it deduced? From (II Sam. 3, 2-5) And unto David were born sons in Hebron; and his first-born was Ammon, of Achinoam the Zezreelitess; and his second, Chileab, of Abigail, the wife of Nebal the Carmelite; and the third, Abshalom, the son of Maachah, the daughter of Thalmai the king of Geshur; and the fourth, Adonijah, the son of Chaggith; and the fifth, Shepliatiah, the son of Abital: and the sixth, Ithream by Eglah, David's wife. These were born to David in Hebron. And the prophet said (Ib., ib. 12, 8) And if that were too little, then would I add unto thee so much more like these, and like these. Now let us see! The number of the wives mentioned in the Scriptures is six. Like this, is six more; and again, like this, is again six more, of which the total is eighteen. But Michal was also his wife, [and why was she not mentioned]. Rab said: "Eglah is the same as Michal. And why was she named Eglah? Because she was beloved to him just as a calf is beloved to its mother." And so also reads the passage (Judges 14, 18) And he said unto them, if he had not ploughed with my calf, etc. But had, then, Michal children? Is it not written (II Sam. 6, 23) And Michal the daughter of Saul had no child until the day of her death? Said R. Chisda: "Prior to that incident she did have a child, but after that incident she did not have a child." But is it not written (Ib. 5, 13) And David took yet more concubines and wives out of Jerusalem. Hence it is to be supposed that he married more? This was to fulfill the number of eighteen. What are wives, and what are concubines? R. Juda said: "Wives are married by betrothal and marriage contract; concubines are wihtout both of these." R. Juda said in the name of Rab: "Four hundred children were born to David by the handsome captives. They had long locks and sat in golden coaches. They were placed with the chief officers of the military, and were the men of power of the house of David." R. Juda said again: "Thamar was a daughter of one of the [above-mentioned] handsome women, as it is said (II Sam. 13, 13) But now, O speak, I pray thee, unto the king; for he will not withhold me from thee. And if she were really his daughter, how could she say that the king would allow a sister to marry her brother? Infer from this, that she was one of the children born of one of the [above-mentioned] handsome women." (Ib. 13, 3) But Annon had a friend … and Jonadab was a very shrewd man. Said R. Juda in the name of Rab: "A shrewd man to do evil."