Kittens need their mother’s milk. However, once they are weaned, some cats are lactose intolerant because they do not have the enzymes to break down the sugar in milk, and cow’s milk can cause diarrhea and upset stomach.
If you do want to feed your cat milk, after you established that it is not lactose intolerant, small amounts of whole milk or branded milk for cats could be explored. Or lactose free milk.
For a healthy cat, exercise, water, and nutritional food are essential and milk could be added supplementally. Water should be available in two locations at all times to main proper hydration.