Shisha tobacco is available in a wide range of flavors, colors and strength. Different tobaccos will also burn at different rates. As a result, there is no one-size-fits-all answer when it comes to how much tobacco you'll need for your hookah. The size of your bowl, bowl packing method, and the type of tobacco you use will all play a role in how much tobacco you'll need.
Most bowls can fit between 15 and 20 grams of tobacco, with some bowls meant for solo sessions only able to fit 12 grams. A general rule of thumb is that once you evenly spread and level the shisha, it should be approximately two millimeters below the bowl's rim so it won't touch the surface of the foil. This allows for proper airflow and prevents the tobacco from coming into direct contact with the heat source, which can result in harsh smoke.
If you want to learn how to pack a hookah bowl, check this tutorial.