For this, we should know:
1. Oxygen delivered = FiO2 x Minute Volume
2. Duration of the transfer
For e.g. If we are expecting a duration of 2 hours (120 minutes) and we are having Size E cylinders and also need to deliver an FiO2 of 1 @ 6L / min
1. O2 delivered = 1 x 6 = 6L/ min
2. Size E has 660 L of O2, which will suffice for 660/6= 110 minutes ; so for 120 minutes, we need 2 cylinders