Question: James Davidson was among the first to report the presence near Stanford of a pair of serial killers credited with killing how many people?

Context paragraphs:
Harpe brothers: Micajah "Big" Harpe, born Joshua Harper (before 1768 (probably, c. 1748) – August 1799) and Wiley "Little" Harpe, born William Harper (before 1770 (probably, c. 1750) – February 8, 1804), were serial killers, murderers, highwaymen, and river pirates, who operated in Tennessee, Kentucky, Illinois, and Mississippi, in the late eighteenth century.  The Harpes' crimes appear to have been motivated more by blood lust than financial gain.  They are most likely the United States' first known serial killers, reckoned from the colonial era forward.  The Harpe Brothers are credited with having killed thirty-nine people, and may have killed as many as fifty.

Cold North Killers: Canadian Serial Murder: Cold North Killers: Canadian Serial Murder is a 2012 Canadian non-fiction book written by Lee Mellor and published by Dundurn Press.  It documents the lives of sixty Canadian serial killers, with the earliest being Edward H. Rulloff and the most recent being Russell Williams.  The book uses Katherine Ramsland's interpretation of what constitutes a serial killer—someone who has killed at least two people on two separate occasions, and who attempted to or likely would have killed again—as outlined in her 2007 book "The Human Predator".  "Cold North Killer's" own definition of what constitutes a Canadian serial killer includes both Canadians who committed murder abroad (such as Keith Hunter Jesperson and Gordon Stewart Northcott) and non-Canadians who committed murder in Canada (like William Dean Christenson and Earle Nelson).

Serial Killers (musical group): Serial Killers is a musical group composed of B-Real, Xzibit, and Demrick.  The group was formed in 2013 and released their first album on October 31 of the same year.  In 2013 Serial Killers teamed up with music video director Matt Alonzo to create a video for their first single, "The First 48".

James Davidson (Kentucky): James Davidson was born in Lincoln County, Kentucky.  He was the son of George Davidson, a captain in the Revolutionary War.  He and his twin brother, Michael, married sisters; the sisters, Lucretia and Jane Ballenger, were granddaughters of Kentucky pioneer and eventual state treasurer John Logan.  James Davidson was among the first to report the presence of notorious outlaws the Harpe brothers near the city of Stanford, their first reported appearance in Kentucky.

My Life Among the Serial Killers: My Life Among the Serial Killers: Inside the Minds of the World's Most Notorious Murderers is a book written by Helen Morrison, M.D and Harold Goldberg.  It presents the cases of ten serial killers, and touches on many more.  Morrison spent hundreds of hours in face-to-face interviews, over many years, with several of the subjects.  She uses the individual stories to explain and put forth her ideas on what makes a serial killer.

Serial killer: A serial killer is typically a person who murders three or more people, usually in service of abnormal psychological gratification, with the murders taking place over more than a month and including a significant break (a "cooling off period") between them.  Different authorities apply different criteria when designating serial killers; while most set a threshold of three murders, others extend it to four or lessen it to two.  The Federal Bureau of Investigation (FBI), for example, defines serial killing as "a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone".

Patrick Kearney: Patrick Wayne Kearney (born September 24, 1939) is an American serial killer who preyed on young men in California during the 1970s.  He is sometimes referred to as "The Freeway Killer", a nickname he shares with two other – separate – serial killers, William Bonin and Randy Steven Kraft.  Kearney may be among the most prolific serial killers in United States history, claiming possibly as many as 43 victims according to law enforcement.

The Killer Speaks: The Killer Speaks is an American documentary television series on A&E that debuted on April 11, 2013 and ended on May 29, 2014.  "The Killer Speaks" features actual convicted felons as they describe their crimes step-by-step in chilling detail.  It's a journey through the minds of killers, an autopsy of their psychology, with the goal of shedding light on what leads people to kill.  The series includes spree killers, serial killers and domestic violence killers.  "The Killer Speaks" casts killers from all over the country, not just from "The First 48" series.

Raya and Sakina: Raya and Sakina (Arabic:ريا و سكينة ), were two Egyptian sisters and considered as Egypt's most infamous serial killers, they, their husbands and two other men began killing women in the Labban neighborhood of Alexandria in the early days of the 20th century.  The police were plagued by increasing reports of missing women.  Common details in the reports included the missing person's sex (all were females), the missing women were known to be wearing gold jewelry, and were known to be carrying a large amount of money.  Another common detail was the report that many of the missing women were last seen with a woman called Sakina.  Sakina was questioned several times because of the reports, but she managed to dodge any suspicions about her involvement.

Hillside Strangler: The Hillside Strangler, later the Hillside Stranglers, is the media epithet for one, later two serial killers who terrorized Los Angeles between October 1977 and February 1978, with the nicknames originating from the fact that many of the victims' bodies were discovered on the sides of the Hollywood Hills.  The police, however, knew because of the presence of multiple distinct DNA traces and the positions of the bodies that two individuals were killing together, but withheld this information from the press.  These two individuals were discovered to be cousins Kenneth Bianchi and Angelo Buono, who were later convicted of kidnapping, raping, torturing, and murdering ten females, ranging in age from twelve to twenty-eight years old.

Supporting paragraphs: James Davidson (Kentucky) Harpe brothers
Answer: thirty-nine