How many royal dukes are there in the UK peerage?
List of dukes in the peerages of Britain and Ireland
The oldest six titles – created between 1337 and 1386 – were Duke of Cornwall (1337), Duke of Lancaster (1351), Duke of Clarence (1362), Duke of York (1385), Duke of Gloucester (1385), and Duke of Ireland (1386).  The Duke of Ireland was a title used for only two years and is somewhat confusing since only a small portion of Ireland was really under the control of England in 1386; it is not to be confused with the dukedoms of the Peerage of Ireland. Clarence has not been used since 1478, when George (the brother of Edward IV) was executed for treason. (However Clarence has since been used as half of a double title, most recently until 1892 when Victoria's grandson (and son of the Prince of Wales), the Duke of Clarence and Avondale, died at the age of 28). The titles of Duke of York and the Duke of Gloucester have both become extinct more than once and been re-created as titles within the Peerage of the United Kingdom. Both titles are reserved for princes (and their descendants). The Duke of Lancaster has merged with the Crown and so is held by the monarch. On 29 September 1397, in an unprecedented move, six dukedoms were created on a single day. None of these titles is extant.