What is the limiting factor? Buildup of CO2?
People need a certain amount of oxygen for their metabolism, you need to carry that much. CO2 effects the blood pH: too little and the body is too alkaline, too much and it's too acidic. So, you need to maintain a precise amount of CO2 and remove the rest. The scrubbers in the space shuttle were able to regenerate the CO2-absorbent material after use, so there was use of power but material wasn't consumed.
Beyond this, you need to control temperature and humidity. The other requirements than atmosphere for crew survival are that you water, feed and shelter the crew, maintain orientation, and maintain a G-force envelope that doesn't injure the crew.