This recipe use 1 liter(1/4 gal.) 20% cream. Try to use heavy cream that's not ultra-pasteurized.
If using raw milk and want longer shelf life, heat milk to 145f(62c) for 30 minutes then cool to 68-75f(20-24c) in a cool water bath. Then add 1/8tsp. of Flora Danica or Mesophile Aroma B culture.
Allow the culture powder to dissolve completely on the surface of the warmed milk and then stir gently for a few minutes. Let ripen for 24-36 hours at room temperature of 20-24c(68-75f) until cream is curdled and thick.
Taste for desired sourness. Whisk the soured cream briefly before placing into a clean container and refrigerate. If you notice any whey on the surface, this can be ladled off.
Keep your sour cream for up to 30 days in the refrigerator.
-Steve Shapson, The CheeseMaker