A sprig of thyme is not a specific amount, like say a tablespoon or a pound. It's really subjective, but the recipe writer knows that. Eyeball the amount you want and go with it. Thyme has a delicate flavor, and it would take a lot to overpower a recipe. If you want to twist our arm, leaves from a normal sprig of thyme would equal between 1/4 and 3/4 teaspoon. However much you choose, select one with lots of leaves on it.
Read more: Our guide to cooking with thyme
The stems of thyme are edible but are typically too woody to enjoy. The leaves are really what you are after, but leaving the leaves attached to the stem saves prep time. You can just remove and discard the stem and any attached leaves after cooking.