The amount of compost to topdress your lawn can vary depending on factors such as the size of your lawn and the condition of the soil. As a general rule of thumb, it's recommended to apply a layer of compost that is no more than 1/2 inch thick. This will help to improve soil health, encourage healthy grass growth, and provide nutrients to your lawn. However, if your lawn has particularly poor soil quality, you may need to apply a thicker layer of compost, up to 1 inch, to see significant improvement. It's important to avoid applying too much compost, as this can smother your grass and cause other problems.