How do i know how much vitamin d supplement should i take?

1000 units?: If your blood vitamin d is normal and you are not overweight 1000 units daily should be enough.

It depends: This depends on sun exposure. In spring & summer if you get some mid-day sun without sunscreen you don't need to take any. Everyone should take d in fall & winter as the sun is not high enough for the rays that produce d to reach us. Most adults need 5-8, 000 iu/day for optimal levels. See http://www.Vitamindcouncil.Org/about-vitamin-d/how-do-i-get-the-vitamin-d-my-body-needs/ for details.