You don't need to purchase vegetable seeds separately in order to plant them. The number that you see when you click on a vegetable patch is the amount of vegetables/fruit that you have already harvested and have available in your inventory.
All you need to do to start growing plants is tap on your vegetable patches, then tap and drag the chosen vegetable icon to the patches and wait until it ripens.
Please refer to the article on growing plants and vegetables in our Help Center for further instructions.