Kutenay's comments and JAK's chart made me think to add a disclaimer: all of my recommendations above are based on being outside and immobile in -20F weather. And I'll bet that once you experience this type of cold, you won't be doing much standing or sitting for any period of time.

If you're moving fast or wearing a pack and walking slow, you can get by with much less. I've gone cross-country skiing in -25F (but sunny) weather with only long johns, German army wool pants, a wool sweater, fleece hat, light wind parka (Patagonia Krushell) and Swix racing gloves and was quite comfortable. But expose a bare hand at that temperature for very long and you'll have a hard time ever getting it warm again without something very thick to wrap around it.

And here's another story: three of us went cross country skiing in -5F weather. We all wore long johns and wool pants, but one of us was afraid of falling down and getting wet from snow clinging to the wool. So he wore moderately breathable waterproof pants as his outer leg layer--even though I said the snow was too dry to stick to anything. After two hours of some average speed skiing, he was horrified to find a layer of ice totally encasing the inner of his shell pants.