1 acre of land can grow 40,000 lbs of potatoes. At 350 calories per potato you that is 14 million calories. If you need 2000 calories a day that is 730,000 per year. So one acre can support about 18.5 people.
Potatoes can be kept in store all year round, but do they provide all the necessary proteins and stuff those 18 people require?
No one food provides everything but potatoes are close. One guy who goes by th name SpudFit ate only potatoes for a year and his health greatly improved.