Milk contains a relatively high amount of carbs, so it’s not typically allowed on a keto diet very often. A single cup of whole milk contains around 12 grams of net carbs, which could easily exceed your daily carb limit, depending on what else you eat that day.
That said, you may be able to more easily fit plant-based milk alternatives like unsweetened almond or coconut milk into your keto diet, as long as you choose options that are low in carbs and added sugars. Always check the label to be sure.