It depends on the setup you want. You can theoretically use every output on the VGAs, which means either 8 or 10, depending on the variants. If you want to use them as a large array, I'm not entirely sure. There may be limitations on how large a display array can be with geforce cards, because NVIDIA wants to sell their professional cards too, and they usually limit the capabilities of the geforce cards in order to make the users buy the more expensive professional cards.
But if you don't need lots of horsepower, just want to drive a lot of screens, you can definitely do that with AMD cards, because eyefinity works just fine with as many screens as you can connect to the cards, and you can set them up as a large array, in any orientation you want.