The amount of Vitamin D you need depends on many factors include ethnicity, geographical location and diet. It also depends on whether someone is deficient (ie blood tests).
The body needs at least 4,000 IU/day in order to maintain a healthy concentration of Vitamin D in the blood. Because most of us don’t get enough sun exposure, the little Vitamin D we get this way, plus food and fortified food sources, totals no more than about 2,000 IU/day. So, in order to meet the body’s need for at least 4,000 IU/day, most adults should take supplements.