"The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information" (wikipedia)
Some of the consequences of this appear in several unrelated domains ...
In presentations you are asked to have no more than 7 points in a slide.
A researcher concluded that assemble more than 7 people in a team and research effectiveness goes down.
In any advertising program, professionals will tell you it takes a least five to seven exposures before someone may act on your ad. (from here)
Here are some other rules of seven,
the value of an item growing at 7% a year will double over 10 years because of the compounding effect. (Dow will hit 16000)
Seven is an acronym for how to build your business and life,
•S – Strategic Service
•E – Energy
•V – visibility
•E – Experience
•N - Negotiation