As a general rule of thumb start with 1 tablespoon per 5-7 gallons of aquarium water. This is a safe dose for all fish and plants including salt sensitive corydoras.
In terms of milliliters, it is approximately:
15 ml of salt for a 20 liter tank (5 US gallon tank)
5 ml of salt for a 7 liter tank (2 US gallon tank)
There are various arguments whether cooking salt can be used in place of aquarium salt. Personally, I use normal cooking salt (non-iodized) and so far it works fine (no fish nor shrimp nor snails have died).
Update: We have created a YouTube video on Aquarium Salt (Dosage, Benefits and Tips!)
Example of a 5 Gallon tank:
Example of a 2 Gallon tank: