How many people's DNA were involved in the compilation of the reference human genome?
tl;dr: In the Human Genome Project, they used the DNA of four people (though one male provided >70% of DNA. The Celera genome was compiled from five people.


  In the [...] Human Genome Project (HGP), [...] scientists used white
  blood cells from the blood of two male and two female donors (randomly
  selected from 20 of each) -- each donor yielding a separate DNA
  library. One of these libraries (RP11 [anonymous donor from Buffalo,
  NY]) was used considerably more than others, due to quality
  considerations. 
  
  [...]
  
  In the Celera Genomics private-sector project, DNA from five different
  individuals were used for sequencing. The lead scientist of Celera
  Genomics at that time, Craig Venter, later acknowledged (in a public
  letter to the journal Science) that his DNA was one of 21 samples in
  the pool, five of which were selected for use.
  
  On September 4, 2007, a team led by Craig Venter published his
  complete DNA sequence,[21] unveiling the six-billion-nucleotide genome
  of a single individual for the first time.)


Source: Wikipedia