When I used to read bread recipes, I feared yeast for good reason: Bad bread recipes. So many instructed me to heat the water to specific temperatures, between 110 and 115 degrees F., and bloom the yeast. Some wanted sugar in the water. Some told me to wait till I saw bubbles. Or they were neurotic about when to add the yeast. Or the amount, 1-¼ teaspoons exactly. Rest the dough in a warm place (because of the yeast), they said, or away from drafts. Or you have to bloom active dry but not instant. Or don't add the salt until the yeast has started its work.
Hogwash, all of it! Truth of the matter is you can play fast and loose with the yeast and you can use any kind you want. I often just measure by eye or throw a couple 3-fingered pinches in.
Yeasts are living microbes that basically feed on the flour, releasing gas and alcohol that leaven and flavor bread. Three types of yeast are available in grocery stores. The most common is Active Dry Yeast, living yeast with a coating of inactive yeast cells (pictured above). I prefer Red Star and SAF brands (same company). Active dry yeast performs well and has a long shelf life, especially if you freeze it (you can freeze it indefinitely).
Fresh cakes of yeast are also available. I love the smell and feel of this yeast but it's less stable, even in the freezer, so I prefer the above active dry.
Unlike Active Dry Yeast, Instant Dry Yeast doesn’t have a coating of inactive yeast surrounding it, so it works a little faster and you need a little less of it. I find this makes the dough rise too quickly, and therefore less flavorfully, than either fresh or active dry, so I avoid it. (If you're a recipe fanatic and want to adhere absolutely to the recipe, fine by me: if so, to substitute one for another if your recipe calls for a kind you don't have, The CIA's New Professional Chef, recommends replacing fresh yeast with 40% by weight active dry and 33% instant.)
But it's all just yeast, and the bottom line is that how much you use is variable. The same dough can take ⅛th of a teaspoon or two teaspoonfuls and you’ll still have bread. One will simply rise much more quickly than the other.
More important than measuring your yeast is paying attention to your dough. It's a living thing until you cook it. Treat it that way. I think of each dough as a unique individual creature.