Getting enough salt is usually enough to keep the other minerals (potassium, magnesium, and calcium) in line, since the regulatory mechanisms for each of these minerals are all inter-linked. Strive for 4-6 grams of sodium daily (including what is already present in your food), since in several studies released in the past several years all agree that this is the “sweet spot” for sodium intake (too much more or less can have bad effects on our health). This amount of sodium translates to 10-15 grams daily of table salt (sodium chloride).
If you are eating whole foods and avoiding the processed products, you will most likely be getting enough potassium, magnesium, and calcium from your food. It then becomes a matter of making sure to get enough salt in order to keep them all in balance. Some people, of course, will need supplements in addition to eating a well-formulated ketogenic diet, but the first step is always to eat right, before worrying about whether supplements are necessary.