Numbers in Genealogy - how many ancestors do you have 

I did a spreadsheet once on HOW MANY great grand parents I might have and just back a few hundred years and the number are enormous 

A good discussion here 

  http://www.bpears.org.uk/Misc/AncestorParadox/

  http://www.bpears.org.uk/Misc/AncestorParadox/ancpdxrv.html

  http://www.bpears.org.uk/Misc/AncestorParadox/ancpdxya.html

 see Pedigree collapse which
 is a term created by Robert C. Gunderson 
http://en.wikipedia.org/wiki/Pedigree_collapse


The Mountain of Names by Alex Shoumatoff (1985). 
http://www.generations.on.ca/genealogy/pedigree.htm

Prince Charles' pedigree has been examined by Gunderson who found that 17 generations back when Charles should have had 65,536 progenitors, he only had about 23,000, a collapse to 35 % of the theoretical.
http://www.barossa-region.org/Australia_2/Interesting-article-are-we-all-related.html