If 1 mole of any gas is kept at RTP in a 50 litre flask will it occupy the whole container or only 24 litres?

What will happen is the pressure inside the flask will be less than the ambient pressure outside the flask. This is no problem if the flask is strong enough to handle the pressure difference. The pressure of the gas itself will still obey PV = nRT, and the gas will occupy the entire volume of the flask.