How much grafts I need to cover this area? (Photo)

You desire a low and broad hairline. You may regret this one day if you continue to loose hair.  I would not recommend this hairline for you because it is aggressive. Filling this area in will require at least 1500 grafts and perhaps more. You have thinning in the areas you still have hair so these thin areas will need grafts. The degree of thinning could push you to 2000 grafts. A higher hairline is much safer and requires fewer grafts. You may also desire a second pass for fullness. Some people get three passes of grafts. 

Thanks for submitting the photograph. Often it is easier to tell what you need in person, but a rough estimate from what I see is in the range of 1200-1500. Hope that helps.