Look at the corner of a yacht sail. You'll see many layers of cloth at the grommet or Inox ring or whatever is used to attach the sail to the rest of the boat. Then you'll see the layers disappear, one by one, as you travel outward from the grommet. When you sew patches to the cloth, try not to run the stitch lines at 90 degrees to the angle of greatest tension. Instead, try to run the seam lines parallel to the tension lines, or at least not much above 45 degrees from parallel.
Most people use more of the cloth the tent is made of.
In most cases, that works fine, and the patch material will probably stretch and age like the main fabric, too. You might eventually reach a thickness where your sewing machine has trouble going through all that stuff, but those areas can usually be hand-seamed with strong thread and a stout needle. At really high-stress areas, where center poles are going through the fabric, I use webbing a lot. It's relatively low-maintenance, extremely strong, and usually used in areas where it isn't too visible. The best type of seam to use is a flat-fell seam.