I've done 3 different combos and I like them all - they're good for different purposes.
1. single layer of insul-brite. I like to do this for oversized potholders - great for things that aren't too hot and big enough to be folded in half in a pinch for something hotter.
2. single layer of insul-brite + single layer cotton batting. This seems like a pretty "standard" thickness for potholders and works well for using them as trivets and for most other potholder needs.
3. single layer of insul-brite sandwiched between 2 layers of cotton batting. This makes a puffy potholder that can really take some heat. Kind of a pain to sew together, but great for things that have been under the broiler.