The Brisket primal comes from the breast section of the cow - on a steer carcass there are two briskets, one from each half carcass.
At retail, this concept can be confusing because a single brisket is made up of two muscles - the flat and the point; At retail, these muscles are often separated.
Not all steer weigh the same at the time of slaughter either. Meaning their primal cuts - like brisket - will offer different weights. A typical beef brisket from a Hot Carcass weighs 30 lbs - however due to the removal of deckle and bones, the overall net weight can range from 8-20lbs.
Meaning, we get two briskets per steer that weigh anywhere from 8-20 lbs - which results in two brisket flats and two brisket points; The flat makes up most of the brisket at roughly 3-10 lbs.
In short, none. We don't get brisket from cows we get briskets from steer.
A cow is a female animal that has had at least one calf.
A steer is a young male cattle that has been neutered. When a male calf is born, it is considered a bull calf. Between three and six months old, the bull calf is neutered - if left intact the bull calf becomes a bull.
Steer are neutered in order to make them more docile and to prevent reproduction. The lack of testosterone also helps to improve things like marbling and tenderness in finished beef - the stuff that's important to folks who do barbecue.
In-short, if the meat you're eating is higher quality - like that of a brisket, you're eating steer.
Cow meat is less desirable for the human market and is typically used in pet food.
This is a hotly debated topic in the barbecue world.
It's long been said that the left brisket is better than the right brisket because steer rest on their left side. This implies that the left brisket is more tender than the right brisket because the steer uses its right leg to stand up which results in more work for the right brisket.
However, when researching the behavioral science of cattle, this doesn't really seem to hold water:
"In conclusion, overall laterality in lying behaviour is shifted to the left in some groups but not others. Eating behaviour has little effect on time spent lying on either side. Cows switched sides between consecutive lying bouts and switching was more likely if the previous bout was either recent or long."Cassandra Blaine Tucker Et al., Applied Animal Behaviour Science, P.125-131
In my opinion, this doesn't matter much. If you were to taste test the left and the right brisket side-by-side, you wouldn't be able to differentiate them.