Celestine Chronicles

The Mystic Knight (4 book series) by Ryan Jackson Pretty much all mages in the series have harems since doing so increases their power. Even the MC's mom, who is a mage, has a reverse harem. Princesses of the Ironbound (6 book series) by Aaron Crash Due to a magical curse on humans women rarely get pregnant and when they do 9 out of 10 times its a girl, so men are rare and harems are common as a result. Celestine Chronicles (6 book series) by Cebelius The world this is set in, is populated by fantasy races who can only have children by mating with the humans who get isekaied there. So harems are common for the humans who can survive it.