This is where the science comes in:
1. You should use enough baking soda to lift the flour in the recipe. This is around 1/4 teaspoon for 120g or 1 cup of flour
2. You should use the right amount of acid which will be neutralised by the soda. For 1/4 teaspoon of soda this is around 125ml or 1/2 cup of acid.
So, if you had a recipe that called for 240g of flour you would need 1/2 teaspoon of baking soda and 250ml / 1 cup of acid ingredient.
However, you can also substitute some of the bicarbonate of soda with baking powder if it doesn't make sense to use that much acid ingredient. When using baking powder you will need 4 x the amount of bicarb, as it isn't as potent.
You can read more about bicarbonate of soda vs baking powder here.