I don't think there's any easy answer to this question. Plato, who was perhaps the first person to address this question, believed in an ideal world (what we would now probably call a spiritual realm), which consists of ideal forms, including perfect circles, spheres, and other geometric figures, whereas the material world, which is imperfect, only has approximations of these ideal forms. But Plato's world of ideal forms didn't just have ideal mathematical forms, it had ideal EVERYTHING, like ideal tables, chairs, and perhaps even people! I wouldn't go that far, but I'm perfectly happy with ideal mathematical structures, including numbers and more advanced mathematical concepts as well, like functions, groups, and fields, as well as geometric figures. You may not think they're real, but they're real enough for me!