If there's less than 50 nmol/l of vitamin D (actually Calcifediol) in your blood, the amount you take/get may be insufficient. That's the only way to find out how much you'd need it as a supplement.
You get vitamin D from the UVB rays of the Sun, and also from fish. If you're of African origin living in USA or Europe, chances are that your natural sunscreen prevents you getting enough sunlight for sufficient production of vitamin D, the higher North the less production. That's the reason why Northern people aren't black, they're mutated to for improved vitamin D production in less sunlight. Eskimos are the only exception because they get all the vitamin D they need by eating fish and seals.