If you do genealogy, and your ancestors were in the USA west of the Atlantic seaboard before the Civil War, odds are you'll find at least one set of first cousins marrying in your tree. There was a brief span of time in the US (1870 - 1930 or so), after doctors started washing their hands and before birth control was popular, when women routinely had 8 - 12 children, most of whom lived to adulthood.
If that happened two generations in a row, one couple could have 8 - 12 children who married and produced 8 - 12 children in turn, giving the original couple 64 - 144 grandchildren.
You may also refer to my answer on this from this link: Two persons are said to be within the "degrees of prohibited relationship"-1. If one was the wife or husband of a lineal ascendant or descendant of the other; or3.
If one was the wife of the brother or of the father's or mother's brother or of the grandfather's or grandmother's brother or the other; or4.
If the two are brother and sister, uncle and niece, aunt and nephew, or children of brother and sister or of two brothers or of two sisters.