Even in England, you would have to consider King Charles III's name. His given names are Charles Philip Arthur George. No family name. Of course, now his name is even a bit longer: "Charles the Third, by the Grace of God of the United Kingdom of Great Britain and Northern Ireland and of His other Realms and Territories, King, Head of the Commonwealth, Defender of the Faith". So, if you think he might sign up for your app, make sure your name field is long enough, and that the "Preferred Name" field can accommodate a space: "Charles III" or "Charles Rex"