How many grafts would I need to fill in this area and reduce my forehead size?

I will assume from these pictures that your hair is coarse. The arrow I placed on your photo shows the location of the middle of the new hairline and a mature hairline shape.  If I am right, then the number of grafts to do this job would be in the range of 1,200-1,300.  If your hair is finer, then it will take more grafts.  It might take 2,000 grafts to get a reasonable result with fine hair. Everyone is different so the amount of grafts required reflect not only the thickness of the hair itself, but the area that has to be covered. This takes an experienced surgeon to do proper estimates.