40 of them, good for you.
I make my mug rugs 8 by 10 inches with insul-brite so they can also be used for hot pads. Also I have the backing 1 inch larger all the way around, bring it to the front and fold the edge under instead of a separate binding and use a decorative stitch on it. That speeds up the process since I don't have to sew on the binding.
Have fun with using lots of colors and prints.