You have to reach a certain number of flowers and tsunamis hits to become a legitimate fiefdom. At least in the old days. If you don't meet one of these conditions, like if you get hit by tsunamis all the time but you only have two or three flowers that will grow, you can't be your own fiefdom.
The most famous fiefdom checker is Han (はん) Solo. He takes his Millennium Falcon and swoops around checking for flowers and signs of past tsunamis and then he'll either say you can or cannot be your own fiefdom. Thanks Han Solo!