In the 1956 article “The Magical Number Seven, Plus or Minus Two”, cognitive psychologist George A. Miller found that the average human can hold 7 ± 2 digits in working memory. Many believe that this is, in part, why Bell Labs determined that phone numbers should be seven digits.
Miller, G.A. (1956). “The magical number seven, plus or minus two: Some limits on our capacity for processing information”. Psychological Review 63 (2): 81-97.