I had 40 cotton shirts to work with (gifts from my brothers) and I easily managed a queen sized quilt out of those, with plenty of fabric left over to cut into strips and squares. That was for the top. I used a good quality muslin for the backing. If you're going to use shirts for the backing, you'd have to double whatever number of shirts you decide is right. I'll bet I could have made that quilt with 35 shirts and still had fabric left over.