Yeast makes more of itself as the dough rises (unlike, say, baking powder). That means yeast quantities in a recipe are not critical: you can always start with less yeast than the recipe calls for, and just wait a little longer for the first rise to happen.
Professional bakers with schedules to meet pay attention to precise ratios, but if you're baking at home and can afford to let the dough take its time, you really just need a tiny bit to get a dough going. For normal home quantities of one or two loaves, one packet (just over 2 teaspoons) is plenty. One teaspoon is still plenty.