At 84"x84" you'll need 14 in each direction for a total of 196 squares total. Half of that = 98 squares.
Conservatively, you probably get 6 suqares out of a length of fabric (most of my fabrics after washing measure about 42")
98 squares divided by 6 = 17 (rounded) So you need 17 lengths @6.5" = 110.5" (divide by 36" = 3 yards)
3 yards would give you exactly what you need but that would not allow for any miscuts, shrinkage in the wash, off-grain, etc. I would buy at least 3.5 yards.