That all depends on whether you're referring to pasture, rangeland or feedlot space. Otherwise this question is pretty well unanswerable.
cows?
It depends where you live. Some places only require 100 acres for 100 cows, others require 1000 acres for 100 cows.
because 1 cow provides only 10% energy so to get 100% we would need 10 cows
100 pigs
you would need 10 cows and sheep and ship 100 things from the cows and sheep hope it helps
Pretty much the same as they do now.
It depends on how large the ranch is. A ranch that is only 100 acres may only have 50 cows; "real" ranches that are over 1000 acres in size will have over 500 cows.
There are about 100 million cows overall in the US.
100 MB.
For how long a period are you feeding them? And what size are these cows, if they are really "cows" at all? Either way, I'll give you an estimate. One bovine will eat 2.5% of it's body weight, no matter if it's a cow, a bull, a steer, a heifer or a calf. Thus, one 1000 lb (454 kg) cow, with or without a calf, will eat 25 lb per day. For 100 cows, that's 2500 lbs of hay per day.
Assuming you are asking based on a practical need to assess space for storage or something, See here.. http://www.upakweship.com/100_cubic_feet_boxes.htm Forgive me if your need was purely academic.
About 100-125 head cows and calves