A trick question - or at least I don't know the full figure - since Prime Ministers of Commonwealth countries ought to be included. But how many Prime Ministers of the UK?
Winston Churchill 1952-55
Anthony Eden 1955-57
Harold Macmillan 1957-63
Alec Douglas Home 1963-64
Harold Wilson 1964-70 and 1974-76
Ted Heath 1970-74
Jim Callaghan 1976-79
Margaret Thatcher 1979-90
John Major 1990-97
Tony Blair 1997-2007
Gordon Brown 2007-10
David Cameron 2010 -
The answer is 12.
A BBC Article about a dinner the Queen had with her Prime Ministers is available here.
