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How much Vitamin C does a human being require?
First a shocking fact:
99% of all species on Earth do not require Vitamin C.

Why?
Most species continually synthesize ascorbate (vit c) in their liver from ordinary blood glucose. The enzyme in their liver which performs this action is called L-gulonolactone oxidase.

L-gulonolactone oxidase

Even Plants synthesize ascorbate in their cells, suggesting that Vitamin C is extremely important to the proper functioning of all life.

Several species including Guinea Pigs, Fruit Bats, higher apes and humans have a genetic defect that prevents the production of this enzyme. All of these species must take ascorbate since their livers cannot synthesize it.

Research has shown that young monkeys require about 12 mg per kilogram of body weight to prevent the development of scurvy and it is generally accepted that the requirement is proportional to body weight.
Vitamin C requirements of Monkeys
Nutrient Requirments of Non-Human Primates

Scaling these values for an adult human suggest that the MINIUMUM requirement to prevent disease for a 50kg human would be 50×12 = 600mg. This again provides a lower limit to the requirement.

The amount for optimum health appears to be 2 -3 times this amount, suggesting a normal human requirements of 1000-2000 mg/day.

Other research indicates that the synthesis of Ascorbate in animals increases dramatically (2x-3x)under stress:
Ascorbate production for rats under stress

This suggests that ascorbate is essential for the body’s proper functioning in stressful situations (including bodily injury).

Therefore, the available research indicates that a human requires between 1000mg and 6000mg of vitamin C daily depending on health, stress and the presence of disease.