The possible number of offspring within 20 generations can be over 1 million. This elaborates on the process.
"Perhaps we should all put a few British history books beside our family history
papers, since everything and everyone in them, up to the reign of the first
Edward at least, will be as much a part of our family history as
great-grandfather's last will and testament." Brian Pears.
Mr. Pears points out that within a few generations all those with British ancestry are related. Given the fact of Roman, Norman, Danish etc migration and conquest, it is easy to postulate extensive European family ties as well, and to extended relationships throughout mankind.
Thanks to "Buck" Wiseman, Louisville, Kentucky for bringing this question to attention.