According to the California ABC's website there are currently 1,491 type 02 license holders in Napa County. The type 02 license, or Winegrower is what a winery holds. To further clarify this does include all wine businesses that are the Alternating Proprietor type. To go one step further it also includes a group who have their licensed winery located at their home address.
How are they able to do this? Here in Napa County there is the option of filing what is called a "home occupation" type of application which allows them to become a winery at their house and make up to a certain amount of gallons per year. This home occupation type of permit is submitted with either the city of Napa business office or the Napa County Planning office, depending on the location of your house.
This option is a fee friendly one (around $100) for those who only desire to make a small amount but would like to be able to make it themselves and legally sell their product. (Though they aren't allowed to have any customer foot traffic to their site)