sprig of thyme dry substitute ratio
Two sprigs of thyme will yield perhaps a tablespoon of leaves when stripped from the stem, depending on the size of the sprigs.

Using the typical ratio of 1/3 unit dried to substitute for 1 unit of fresh herbs, you would then want about a teaspoon for dried thyme.

Thyme--or herbs and sprices in general--usually does not contribute to the chemistry of a recipe, just to its flavor profile, so you can adjust the amount as you desire to match your personal preference.

You also want to consider how the thyme is used in the recipe.  If it is in a bouquet garni, it would eventually be removed from the sauce or stock or other food product.  If the appearance is critical, you might not want the flecks of thyme in the dish, so you might want to use a spice ball (I would just accept the flecks, as I personally don't even own a spice ball).