I had to look...
Found it here - http://www.learningace.com/doc/4869508/751368280f72cd471538c519a709b19a/1276114621
500 to 700 bags per mat!
But yeah, it would certainly work. Not sure how the warmth compaires to other mats though. My wife makes rag rugs ( among a very great many other things ) and has made a few rag rugs out of the same plastic shopping bags. I'm not sure, but I think she uses the bags whole, and simply braids three of them into a rope. She may be using more than three in the braid because they are quit thick, I'd have to ask her.
Anyway, then it's the same as any rag rug, coiling the long braid and stitching it together with a big needle and stout string like masons twine.
The few she has made were quite thick and soft ( perhaps an inch thick ) and I'd have no trouble sleeping on something like that at all. They are however, very bulky!
The ones she has made were given away to family and used as bathroom mats because they are water proof!