She’ll take on the title given to her by the Queen
With a very few exceptions, women who marry royal heirs take on their husbands’ titles and are awarded the HRH (Her Royal Highness) designation. The Queen always has the final say.
Sarah Ferguson, who married Prince Andrew, became the Duchess of York. Sophie Rhys-Jones, who wed Prince Edward, became Countess of Wessex (and was said to be disappointed she didn’t merit the title of “Duchess”). The Queen awarded William and Kate several different titles, but they are always referred to as the Duke and Duchess of Cambridge.
The odds-on favorite for Meghan and Harrys’ new titles: the Duke and Duchess of Sussex. There has been one notable exception in recent years: Camilla Parker Bowles, who adopted the title Duchess of Cornwall when she married Prince Charles. Although she legally became the Princess of Wales, which had been Diana’s title, it was unthinkable for the prince’s former mistress to taken on the same designation. There has been much speculation about what Camilla’s title will be when Charles becomes King. After the couple’s wedding, the Palace announced that when Charles ascended to the throne, Camilla would be the Princess Consort, not Queen Camilla. But royal insiders have told me Charles is determined to have his longtime love be known as Queen Consort. As monarch, he has the power to award the title to his wife—even if the majority of British subjects are said to oppose the idea.