A Dachshund that weighs 20 pounds can essentially be fed 1 cup of high quality dry food per day. You can break their meals up into half cups, so that they are eating twice per day just as their humans.
The recommended amount for this breed is around 1/2 cup to 1 1.2 cups of dry food a day.
A lot of how much your dog will eat daily is contingent upon their metabolism, age, and activity rate. Always go by the instructions from the packaging or consult with your veterinarian.
You should include a protein source such as white meat chicken or some kind of beer or liver.
As always, you should readily have available fresh drinking water for your Dachshund.