If you have good grass, you can use a couple of cows per acre. You ideally should have an acre per cow and for both cows and goats you should have at least two each, three is better. They are not happy being single cow or goat. Goat is the one animal that I haven't raised but my friends tell me they maybe smaller than cows, but you have to have higher (much higher) and denser fencing, along with a lot of things for them to climb around on. Both can be escape artist, but goats are very nimble. Beware of anyone wanting to sell you a single cow or goat out of a herd, that usually implies they are troublesome escapers. it's not worth the "great deal" they are offering you. i learned this the hard way after buying a single cow that escaped from the back of my trailer at 60 mph, yes I lost her and the baby she was carrying) Once you have a security fence like Ft Knox, goats are easier to handle, from milking, feeding, trailering, houseing and birthing. Also, ideally you should have one donkey, with either one. They are the best alarm system you can have. They will make a lot of noise if anything gets into the fence as well as escaping out.
Rabbits and chicken are some of the easiest animals to raise! They produce a lot of babies easily, grow quickly for the meat and take up the smallest place on a farm. You need good housing first and you can also put in a big run (make sure your fencing goes at least a foot and a half under ground) They eat just about anything that you will normally put into a compost,(including they love yard, leaf and grass trimmings) while giving you some of the best fertilizer for your garden. (and their crap is smaller and easier to set aside for fertilizer) Plus, cleaning up after them is a lot easier. Vet bills and medical are a lot less exspensive than goats or cows. AND...they are a hell of a lot cheaper to buy than goats or cows.
There are books out by Storey press all about farming, raising animals and self-sustained living. My first advice to you is to make sure you have good, solid fencing/housing for the animal you are wanting to raise and learn how to repair it yourself. It will envolve some special tools, but it will save you money for someone coming out to fix it.
As usual, I wrote a book.... Anyways, I can't tell you how great it is to have your own animals and veggies. They are a lot of fun. I hope that you will start with chicken and rabbits because they are cheaper animals all the way around. Plus, if you happen to screw something up with them, you are not in the money hole as much. I know that sounds bad, but nothing is worse than paying several hundred dollars on a cow and then losing them to illness or injury. You can buy chicks and rabbits for a few dollars each. If you are wanting them for food they are a lot less exspensive to care and feed.
I envy you and hope that you guys love the little farmlife. It is so rewarding!! Good Luck.