They need 3 serves of dairy a day if this is their only calcium source (which it most likely is not). This includes all dairy, and obviously with a balanced diet they will get calcium from other sources so 3 serves of dairy are not necessary. There is no minimum amount of milk - they don't need any if they get calcium elsewhere, but I wouldn't give more than 3 cups a day as it will probably interfere with solids intake if they have more than that. HTH.