Apparently much less than we think we need. And an even more bizarre paradox is that it is the carbon dioxide that our lungs produce and we expel as soon and as much as we can, that practically helps to absorb oxygen and utilize it in the body. Carbon dioxide is the fundamental nutrition of every life form on Earth. It acts as the main regulator of all functions in the organism, as it is the main internal environment, regulating activities of all vitamin and fermented substances of the organism. It is the vitamin of all vitamins, and is the basic food product of all life forms on Earth.
Plants absorb carbon dioxide from the air.
Animals feed on plants and humans feed on both of these. That means that it would be much more veneficial for us to keep carbon dioxide in our body longer and in greater quantity. And that leads to the conclusion that deep breathing is not all that good for us, as we were conditioned to believe for generations. Breathing deeply, we inhale a lot of oxygen which we can't absorb anyway, and at the same time when inhaling, expel too much carbon dioxide, which could have helped us to absorb more oxygen and give more of it to our tissues. It can be easily proven by trying to breathe elaborately deeply for 2-3 minutes.