>>439689That's for the whole kit, though. Which is reasonable for winter camping. In any other season or in warmer areas, you can strip it down to about six.
OP, Rectangles are more comfortable, with more footroom. Mummies are warmer, as they have less air space that your body heat has to process. If you CAN sleep in a mummy, they are always a better choice for use as a sleeping bag.
If your looking to DIY a cheap underquilt, a rectangle is a better choice.