You can adjust the amount of milk depending on how thin or thick you like your dressing. Just be aware that it will thicken up a bit after sitting in the refrigerator for a while, so take that into consideration when you're making it.
If you want to make a ranch dip instead of dressing, you can cut the amount of milk way back and use the dip for other Farm Rich Appetizers like Mozzarella Bites or Jalapeno Poppers!