There have been a couple of occasions where I had some stuffing leftover. It all depends on the size of mushrooms and how much stuffing you can fit into them. If you want to make sure to use up all the filling, you can get an extra pack of mushrooms when shopping for the ingredients, just in case.  Otherwise, there is a very simple and delicious way to use up leftover filling. Spread some filling on baguette slices, flatbread triangles, or crackers. Bake it for 10-15 minutes at 375°F, depending on how much filling you add to each slice.