Depending on house size, stove efficiency, and how much they are home during the day, around 4-6 here. I cut firewood for my parents, MIL, and some for my BIL. MIL and BIL have back-up sources of heat so it isn't a make or break deal for them if they run short. Their amounts vary from 3-7 cords for the 3 houses. One big factor in wood consumption is if they are retired and home during the winter days or still working and gone from the house. Keeping the house warm all day burns a lot more wood and older folks usually like the house warmer.
My dad always likes to have a two-year wood supply on hand. His thought is if a cold winter hits, your going to be okay or if a guy gets hurt and can't cut firewood one year, you're still okay.
Growing up, we had a 3 story farm-house, poorly insulated, and we would use 10-12 cords minimum with no back-up heat source. Cutting wood was just as important as going to work, taking care of the animals, etc..