If you received an RX from your doctor, then it is most likely,
20mEq (milliequivalents).... this is not milligrams. It has about 800mg of elemental potassium in each tablet.

You can get the same amount of potassium in a 12oz V8.

That chart on your link gives the milligrams (mg) first, and mEq in the second column.

The goal today from nutritionists is to consume 4700milligrams of potassium containing foods a day. Some anthropologists suggest our hunter gatherer ancestors consumed more and perhaps up to 7,500 mg a day from whole foods.

Diuretics also deplete magnesium, so taking some within RDA levels would be good insurance for you. Very low magnesium levels can lead to heart arrhythmias.

People can waste potassium from diarrhea, and also certain types of kidney damage. So if you test low in spite of the RX potassium, some further testing and examination by your doctor would be a good idea.