Well if you can bring them into a space that was heated before power loss I would think fewer then if in a garage.
I lean toward the good ole hand warmers like these...
I understand your concerns.
I am brooding chicks in my shop (unheated) and had to put a piece of 2" thick insulation board over them when temps dropped to single digits here.
My brooder is about 30" wide and 6' long. The water is on the opposite end from the heat lamp and actually froze once...