Then your vet is an ****.Introducing a second cat usually does more harm than good. Our vet strongly discouraged us getting a second cat as cats are solitary hunters so usually want to be alone. The exception to that is usually with siblings.
We have been introducing cats of all ages into our house in ones and twos over the last four decades. We've always had between two and five at any one time. The ONLY issue we ever had was when we temporarily fostered my daughter's two cats - they and ours didn't get on particularly well but nothing too dramatic.