The 9 X 12 is 9 squares wide by 12 squares long = 108 squares.
So you need 54 print squares and 54 solid squares. There are 40 to 42 10" squares in a layer cake.
If you wanted to add a border, you could make your center 8 squares wide by 10 squares long. Then add a border to make it the size you want. That would be 1 pint layer cake and one solid layer cake.