Why 7 +/- 2 people in a Team ? Accordng to the article mentioned at the bottom this is because of the fact that people can only remember 7 +/- 2 thing in their short term memory.
A common model of human memory divides it into two units; a short term memory and a long term memory. The short term memory is a limited capacity store for holding temporary information. The long term memory is usually treated as an infinite capacity store capable of holding information throughout a persons life. Information is held in short term memory before being transferred (or not) to long term memory. The 7±2 urban legend is that the capacity of short term memory is seven plus or minus two items of information (some people being able to only hold five or six items, while others can hold eight or nine). The extent to which short term and long term memory really are different memory systems, and not simply two ends of a continuum of memory properties continues to be researched and debated.
The value 7±2 as a measure of short term memory is an urban legend. It only applies to speakers of English attempting to remember a sequence of digits. Actual human memory performance depends on many factors and cannot be approximated by a numeric value.
The above quotes come from this article : “The magical number seven, plus or minus two: Some limits on our capacity for processing information”, from George Miller in 1956