Regnal numbers are ordinal numbers—often written as Roman numerals—used to distinguish among persons with the same regnal name who held the same office, notably kings, queens regnant, popes, and rarely princes and princesses.
It is common to start counting either since the beginning of the monarchy, or since the beginning of a particular line of state succession. For example, Boris III of Bulgaria and his son Simeon II were given their regnal numbers because the medieval rulers of the First and Second Bulgarian Empire were counted as well, although the recent dynasty dates only back to 1878 and is only distantly related to the monarchs of previous Bulgarian states. On the other hand, the kings of England and kings of Great Britain and the United Kingdom are counted starting with the Norman Conquest. That is why the son of Henry III of England is called Edward I, even though there were three English monarchs named Edward before the Conquest (they were distinguished by epithets instead).