It is commonly believed that Jack the Ripper murdered five women - often referred to as the "canonical five".
The first mention of the number of victims being limited to five was made by Melville Leslie Macnaghten (appointed Assistant Chief Constable, Metropolitan Police in June 1889, promoted to Chief Constable in 1890) in a memo of 23 Feb 1894
(now in the National Archives - Metropolitan Police files MEPO 3/140; first made known to the public in the book The Complete Jack the Ripper by Donald Rumbelow, published by W H Allen in 1975):
the dreadful crimes so placidly perpetrated in Mitre Square and Berner street, I conceived an ardent desire to visit and see for myself the region... .
"Now the Whitechapel murderer had 5 victims - & 5 victims only - his murders were
  (i) 31st Aug `88.    Mary Ann Nichols, at Buck`s Row, who was found with her throat cut & with (slight) stomach mutilation
  (ii) 8th Sept `88.    Annie Chapman - Hanbury Street, throat cut, stomach & private parts badly mutilated & some of the entrails placed round the neck
  (iii) 30th Sept `88. Elizabeth Stride, Berner`s Street, throat cut, but nothing in shape of mutilation attempted & on same date
                             Catherine Eddowes. Mitre Square, throat cut, & very bad mutilation, both of face & stomach
  (iv) 9th November. Mary Jane Kelly. Miller`s Court. throat cut, and the whole body mutilated in the most ghastly manner".

Macnaghten (appointed Assistant Commissioner [Crime] in 1903, knighted in 1907, retired in 1913) re-iterated his claim of five victims in his memoirs in his book Days Of My Life, published by Edward Arnold in 1914:
     Suffice it, at present, to say that the Whitechapel murderer committed five murders, and - to give the devil his due - no more,

The term "canonical five" is credited, by researchers and authors Paul Begg and John Bennett, in Jack the Ripper: The Forgotten Victims, published by Yale University Press in 2013, to have been coined in 1987 by Martin Fido, researcher and author of The Crimes, Detection and Death of Jack the Ripper (Weidenfeld and Nicholson. 1987).