Pat,
A typical gallon of Behr paint will cover about 300 square feet of area, depending on how dry the old finish is and if it is smooth or textured wood. How wide is the exposed fascia board? If it is a 1X6, you will have about 60 sq feet of area and a quart should be adequate for one coat. You can multiple the width of the board in inches X's the length of the board in inches and divide by 144 (the number of inches in a square foot) .
For example:
120 feet x 12 inches equals 1440 liniear inches, times 8 inch wide fascia equals 11520 square inches , divided by 144 (a square foot in inches) equals 80 square feet. In this example, you would be on the borderline of needing one quart.
Hope this has helped