One extra battery per vehicle sounds excessive. I have a few extra batteries on the shelf, plus several battery chargers and maintainers. If the temperature drops to 20 or 30 below 0 F. The plan is to get at least one vehicle started. Then drive around to the others as needed and jump start them. Also we have oil pan heaters on ones close enough to an outlet. When you live in Montana you learn never to skimp on batteries or studded snow tires.