Different recipes give ratios of baking soda to buttermilk that vary by two to one. Since buttermilk has long been a standardized manufactured product, there should be only one ratio. Do you know what it is?--Earl
If the recipe contains no other acidic ingredients, then the ratio should be about 1/2 teaspoon of baking soda per cup of buttermilk. I say 'about' because there is a bit of latitude in the ratio before the quality of the final product is affected noticeably.
That said, though, there are a number of other ingredients typically used in baking, such as molasses or un-dutched, natural cocoa, both of which are acidic and will react with baking soda. If any acidic ingredients are used in the recipe in addition to the buttermilk, then the total amount of soda needs to be increased accordingly. The exact measure will depend on the ingredient and amount used. Also, different types of the same ingredient, for example dark molasses vs. fancy molasses, will vary in their acidity and therefore the amount of baking soda that they will react with.