If it’s really cold, two puffies goes a long ways. For bulk purposes, I like stacking a down puffy over a synthetic. Heated socks + some insulated booties. You can put the socks on low and the batteries will last quite awhile. Bring a thermos with a warm drink, sip it slowly, and you should be set.
I spent a couple of days during the late season last year sitting in the shade line under the north face of a peak at 10,000 feet and it was frigid, but I stayed quite toasty using the above combo.