Mary's Babies by Maud Dromgoole
Mary Barton, a pioneer of fertility treatment, thought her husband was perfect. And doesn't every child deserve the perfect father? So Mary used her husband's sperm to impregnate up to a thousand women, and then burnt all the records. A thousand resulting children, the "Barton Brood," with no idea about their shared father. Meeting each other. Making friends. Having babies.