Use this calculator to figure out how many ancestors you could have in your family tree. Use the sliders to change the number of generations, and the average number of children in each family (that have married and have children). Some variables are impossible to compute but this calculator will give you some idea of the numbers.
Formula used:
{21 + 22 ... 2n } x ( 1 + 4(x-1) + 4x(x-1) )
where n=generations and x=children