I pop about 1/2 cup of kernels, and it makes about 6 quarts of popcorn. It really varies depending on the brand you use, so you may need more or less.
You'll notice the brown sugar mixture start to foam up when you add the baking soda. That just means your baking soda is good.
Each time you stir the popcorn and caramel mixture it becomes more coated with caramel, so it's important not to miss a stir.
Make sure to get the bottom of the pan when you stir the popcorn. Lots of good stuff is down there!