Every 20 generations could equal over one million different people. However, due to the custom of past generations marrying within the family it doesn�t really work out that way. You would always have the numbers below for each generation,  but some of the people would be duplicates as when 2 first cousins marry, they share the same grandparents.

If you figure 30 years per generation, which in my opinion is more realistic, 20 generations represents about 600 years.  MaryCarol
 

You Have.....

2            Parents
4            Grandparents
8            Great Grandparents
16           2nd Great-Grandparents
32            3rd Great-Grandparents
64            4th Great-Grandparents
128           5th Great-Grandparents
256           6th Great-Grandparents
512           7th Great-Grandparents
1,024         8th Great-Grandparents 
2,048         9th Great-Grandparents
4,096         10th Great-Grandparents
8,192          11th Great-Grandparents
16,384        12th  Great-Grandparents
32,768        13th Great-Grandparents
65,536        14th  Great-Grandparents
131,072      15th  Great-Grandparents
262,144      16th  Great-Grandparents
524,288      17th  Great-Grandparents
1,048,576   18th  Great-Grandparents