There is a boatload of information out there on this topic. Five minutes on Google rendered link below. Note difference between #6 and #8. There are probably better links out there. Best research begins by asking the right question, correct?
Naturally, this changes depending upon the formation. This example is 4-2-3-1, the most common modern formation.
http://www.football-bible.com/soccer-info/jersey-numbers.html
Let me know what you think. I would rather discuss these topics than do just about anything else, including my job. #soccernerd.