How much land do I need for a milking goat?

Usually recommended is 4-6 goats per acre, depending on pasture. Goats are browsers, not grazers, and require roughage usually in the form of good hay or meal to supplement good pasture.
You could have them indoors and bring all the food to them - grass, tree fodder, hay, silage, meal etc.
They also need plenty of good clean water.