Please see this post for my responses to the comments on royal surnames:
Titles of the Dutch Royals
In Britain and America, the term "middle name" is used to mean any name that does not go first or last out of all of the individual's legal names, and a middle name may be a Christian/personal name (e.g., Boris) or a family name/surname (e.g., van Oranje). (This differs from the term "middle name" in Scandinavian cultures, where it is strictly used to mean a family name or name analogous to a family name, such as a patronymic.)
It would sound quite unusual to use De Pfeffel as a Christian name, so I would instead assume it is being used as a family name. However, the Anglo-Saxon naming convention is to use only the first and last of the legal names as the short version, provided that the names are not hyphenated (as hyphenated names are often treated as a single name). In the UK it would be usual that Alexander Boris De Pfeffel Johnson would use Alexander Johnson and Eloise Sophie Beatrix Laurence van Oranje-Nassau van Amsberg would use Eloise van Amsberg.
In the Netherlands and Belgium, on the other hand, it is more usual to use the first of the personal names and the first of the family names, which for the legal names above would be Alexander De Pfeffel or Eloise van Oranje.