It depends a lot on the design and size you are looking for. 15 large long sleeved shirts would probably be adequate for a nice twin. Figure about 1/2 to 3/4 yard average useable fabric per shirt. If you are seeking long lengths for strip piecing or 8" squares you'll need more, vs smaller squares. I just buy 100% cotton shirts as I see them for $1 - 2 each. The larger the better, prefer long sleeves without worn elbows for maximum value. Definitely wash them well before cutting. Not a clue how many for a quilt as I've only "deboned" them as Bonnie Hunter calls it. I have about 4 bins worth waiting for me to cut them down into useable units. I love scrappy quilts with maybe 1 unifying element or color so I don't get dizzy looking at it.