It depends on what you consider a "mistake". If it is a tiny "whoops", probably an average of 10 per quilt. If it is an "OH NO!" (w/o tears) then more like 5 per quilt. If it is an "OH NO!" with tears, then only a couple per quilt. If it is a real mess that requires my sweetie to come into my sewing room, calm me down, help me figure out what to do now, then only once every 2 or 3 quilts. When you ask a question like this you need to define your terms more exactly! Mistakes are why appliques exist!