I've done 12--used Pellon 911FF (I find that some t-shirts are very thin and stretchy, others not so much so this makes them all stable, have also used French Fuse but it does have some stretch so you have to make sure the direction of it--I fine the Pellon cheaper in long run). Bree123 gave you good advice. I would nix the idea of flannel on the back--once you have the t-shirt's stablized and either sew to sashing or to another stablized t-shirt the seams are heavier than when it's just seams of 2 cottons fabrics--the bat will help keep the bulkier seams from being felt so easily on the back. I'd go with wool if you think a cotton will be too warm--it's more expensive, but probably worth it for your quilt. I'd definitely do some FMQ across the whole quilt--that way you can go around/through the logos of the t's in a way that doesn't distract but still gets the quilt spacing correct (about every 4").