Most t-shirt quilts are made by ironing a fusible interfacing on the back of the t-shirts to stabilize them. The squares are then sashed with regular quilting cotton. If you are trying to sew regular unprepared t-shirt squares to each other, you will need to use a knit overlock stitch?
I am not understanding the number reference. Are you trying to sew appliqué numbers to the t-shirts? If so without the fusible behind the t-shirts you are going to get stretch.