On a recent Belgian Dark Strong I used 2 whole stars in 5 gallons. It is not too much, but it is close to too much. A good friend who tastes my beer critically did not pick out the flavor immediately, but as soon as I mentioned it, he didn't have to go 'hunting for it' either.
So I guess I would say it depends on how big the rest of the beer is. If lighter, I'd go with one - if heavier I'd go with two.