Here in Wales the grazing is a bit poor, so its generally considered 2.5 sheep per acre, possibly more in the summer months. It could very well become heavily burdened with parasites too. You might be better putting them on your neighbour's field. I know nothing about goat keeping, but would assume the situation would be similar. If you decide to go ahead, consider the nature of the breed you chose. Some of the older breeds do have a habit of trying to abscond as soon as the grass starts to diminish and they can be very persistant at getting out of places, but then I suppose if you are considering goats, your fences must be pretty good.
All the best.