If you had to survive on the fewest number of foods, for a few months at most, which foods and how many?
Grain amaranth provides protein, some fat, plenty of carbs, calcium, magnesium, phosphorus, potassium, fiber, zinc, selenium, iron, copper, manganese, and some vitamin E. You might need 500 grams of grain amaranth per day as your main staple, but it’s probably the best staple food bar none.
You would need 3 to 5 grams of salt, since sodium chloride is an essential electrolyte. You would literally die without salt in your diet. Iodized salt is best.
Grain amaranth does not have enough dietary fat, so you would a vegetable oil with both essential fatty acids (omega-6 and omega-3). Canola oil would be best. Soybean oil is a close second. You would need at least 5 tablespoons of vegetable oil per day.
So that is three foods: (1) grain amaranth, (2) vegetable oil, (3) iodized salt. You won’t be getting all the vitamins and minerals you need, so (4) a good multivitamin supplement would also be important.
You could survive for a while with just those four things. The diet is not recommended. It’s a thought experiment only. Amaranth is about 13 to 14% protein, so a supplementary source of protein would also be helpful, like powdered milk, or seeds, or nuts.