With goats there is no accurate formula...............it does depend a great deal on the quality of ytour land.
Also whether you house the goats at night............most of us do.
Bear in mnd, goats don't eat grass like sheep.............possibly 40% less as they prefer 'rubbish' i.e. weeds, hedges etc.
I have grazing across 14 acres, 40 goats & 4 horses and we still have plenty of grass. The topsoil here on Dartmoor is thin so cannot be classified as good ground for grass.