Butter contains about 80% milk fat, and its source ingredient, fresh cream, contains about 40% milk fat. So fresh cream is required twice as much as butter.
Furthermore, if the milk fat content of milk is about 4%, it requires 10 times as much as fresh cream, that is, 20 times as much as butter.
Therefore, to make 100g of butter, you need 20 times more milk, or about 2L of milk.
In practice, the milk fat content of butter changes over the seasons, with a yearly average of 3.7%. So, to make 100 g of butter, on average it will take slightly more than 2 L of milk (to be precise, 2.2 L).

As the milk fat content of butter changes over the seasons, the amount of milk required to make butter will also change depending on the fat content of the milk.