When King Daron Targaryen died the crown went to the oldest son as it was the law. No daughters from the king would get the iron throne. That law was made in the era of King Balerion Targaryen. To prevent war between family members.
Prince Aegon known as Aegon 7th of the six kingdoms was in the very center of life. Aegon's brother, Viserys was an heir if his brother Aegon had no sons. But Aegon had his own ideas in mind. One was that his brother would never sit on the iron throne. Aegon would not give him such an opportunity for it. So Aegon called in his lords and the hand of the king on who he should marry. He needed a queen fast. Almost all of the small council said Lady Cassanna Hollard.
That she is from good family and that she is good stock, she should produce many sons for Aegon. Aegon was all for it. He had her called to the capital where he married her on the spot in king's landing. The people of King's landing were happy to have a queen and waited for the royal children to be born. And they did have children but Aegon acted again and taking another wife for himself. He chose another lady from a good family to be his wife, and second queen.
His advisors were confused on why he took another wife. They tried to stop him from it but Aegon did it anyway. She is Lady Leonore Velaryon, As people call her Queen Velaryon. Since the king had two queens everyone thought that the king was happy and that he would keep his two wives but no. He picked another one, Lady Olive Osgrey from the Reach as his next queen and third wife. The next wife he took was Lady Maeve Brax from the Westerlands as she became his fourth wife and a queen. King Aegon did not stop after that, he married again to Lady Mara Sunderly. By this point his small council, at least his hand knew what the king was doing. On why he was marrying so many ladies. These ladies are specific, the reach, the Westerlands, the hand of the king could see what Aegon was during.
It was complete madness but smart in a way.
After marrying Mara Sunderly, Aegon took princess Sarrella Martell has his 6th wife and next Queen.
People were getting frustrated with the king on marrying so many times when each of them are still alive. But they could not complain to much, it could be worse. Thank goodness that the Targaryens are not marrying each other anymore.
Ageon Targaryen wanted more wives, he was looking for each ladies that are keys to each lands. The next one Aegon chose was Lady Joan Lynderly. Coming to seven wives, one would think that Aegon would be happy but nope. There was one more key he needed. The north.