When stripped from the stem two sprigs of thyme will yield around a tablespoon of leaves, however this does depend on the actual size of the stem and the sprigs. If you use a typical ratio of ⅓ of dried thyme to substitute a singular unit of fresh herbs then you will need around a teaspoon of dried thyme to equal the amount that you want.
Do remember that any herb does not really add anything to the chemistry of the recipe overall it just contributes unique flavors, so you can always adjust the amount as you cook. This is why it is better to start off small and do taste tests as you cook until you reach the desired flavors. This is the best way to judge if you are substituting fresh herbs for dried herbs. Check as you cook, add more if you need to. This will often result in a more unique dish than if you blindly follow the recipe.