If you are looking at chapter 11 in NFPA 99, they have 3 sections based on the quantity of gas involved. 

There are rules for up to 300 cu ft of gas, between 300 and 3000, and over 3000.

The air systems I have typically seen are a 2x2 cylinder manifold in a ventilated cylinder storage room. 

I have seen at least three installations where the medical air manifolds at a non-standard delivery pressure were not chosen for the medical air delivery.

Instead, each of these installations had a medical air H cylinder of 230 cu. ft. each cylinder restrained to the wall near the chamber and connected via a regulator and flexible hose to the chamber medical air connection. Each chamber was in it's own room so each room had less than 300 cu. ft. as described in Chapter 11. We chose to have the regulators each equipped with a "T" on the regulator outlet with a 100 psig pressure relief valve for protection of the patient in the chamber.