How many royal dukes are there in the UK peerage?
List of dukes in the peerages of Britain and Ireland
31
This is a list of the 31 present and extant dukes in the peerages of the Kingdom of England, Kingdom of Scotland, Kingdom of Great Britain, Kingdom of Ireland, United Kingdom of Great Britain and Ireland, and the United Kingdom of Great Britain and Northern Ireland 1927 and after.  For a more complete historical listing, including extinct, dormant, abeyant, forfeit dukedoms in addition to these extant ones, see List of dukedoms in the peerages of Britain and Ireland.