Answer
The answer is four. Louisa Ulrika was the sister of Frederick the Great and the mother of Gustav III of Sweden. She had a total of fourteen children, but only four survived to adulthood. These included her son Gustav, who succeeded to the Swedish throne, as well as her daughters Sophia Albertina, who became the Abbess of Quedlinburg, and Hedwig Elizabeth Charlotte, who married the Duke of Holstein-Gottorp.