I presume this reference is to the book "Births, Deaths and Sponsors 1717-1778 from the Albermarle parish Register of Surry and Sussex Counties, Virginia" by John Bennett Boddie.
There are a William and Martha Mangum, whose son James was born 2/2/1734.
We also find a William and Mary Mangum, who had a son William on 5/16/1736; a daughter Sarah born on 10/14/1743;
a son Arthur, b. 5/2/1744.
There is also a couple, William and Mary Mangum, who had a son Henry, born 1/24/1773. It is unclear whether the parents of William, Sarah and Arthur, were the same couple as the parents of Henry, born almost 30 years later.