If you only have a light weight backing, use several layers of backing. Four is not too many. You will have to use some common sense judgment on this. Sometimes multiple layers of backing is better than using one layer of a heavier backing. Just as the number of stitches determines the quantity of backing, the fabric type will determine the type of backing you need to use.
Common sense is also a bonus here as well.
Sometimes if your stitch count isn't too heavy you may be able to get by with a quality tear-away on knits. One thing you do want to make sure you check is that your backing doesn't stretch. Some backings may not stretch vertically or horizontally but may stretch diagonally. If you have any of this type of backing, use it for insulation in your walls. That's about all it's good for.