sorry- and about 1 tablespoon per cup. It depends on how thick you want it!! You can also simmer it and evaporate some of the liquid to make it thicker. "artificial" thickeners are usually not neccesary in legume soups
I do sometimes in a pinch... disolve 1 Tbsp cornstarch with a little bit of cold water and stir until smooth, mix in and let it do its thing, depending on how much soup will depend how much cornstarch you need, just make sure you dissolve it first.
I have found, however, that a flour and butter rue makes a bettter, albeit more fattening, thickener for soups and sauces.
for lentil soup I would puree some of the lentils to thicken it or use a flour slurry ( Mixed with water) Cornstarch works that way too. fBoth will thicken upon a boil temp. cornstarch is clearer and flour is cloudy. Other starches can be used too. Arrowroot is one that should be used spaingly, iti s very strong.