I never heard of 10ft. wide fabric. As I remember, it is usually 42", 45" or 60" wide.
How much you need for your sofa depends on the areas to be covered. Does it have pillows, does it have a skirt, is the under cushion area covered in same fabric? You would have to sketch each area out on paper & include the measurements & then map all the pieces out to see how much yardage you would need for each needed piece.