We really can't answer that without knowing how big you want your lap quilt to be, and what size squares you will be using for your 9-patch.
Let's assume you want 3-inch finished squares in the 9-patch. The 9 patch blocks will be 9" finished, and your d9p will then have the same number of 8" blocks. If you make the quilt 8 blocks wide and 9 blocks long, it will finish (before borders) at 64x72 inches, which is a nice lap size. This means you need 8x9=72 blocks, which means 72 3.5" squares of each fabric.
In one 40" strip of fabric, you can get 11 3.5" patches, so you will need 7 strips. 7x3.5" is 24.5", so you need 24.5" usable of each fabric. You might get by with 3/4 yard, but I would buy 1 yard of each to allow for shrinkage, squaring up, and possible mistakes.
You may also want border(s), and you certainly need backing and binding.
Now I've just told you to buy more than 9 yards of fabric for a lap quilt. You see why quilting can get expensive! If you want to make the 9 patch squares bigger than 3" you will need less fabric. Just pick a size and go through the logic above.