Oxygen is prescribed by a target saturation rather than a specific flow rate.
According to the BTS Guideline for Oxygen use in Healthcare and Emergency Settings this is:
This is currently due for review, so keep in mind these target saturation ranges might change. During the Covid-19 pandemic targets were 92 - 96% and there seemed to be no negative impact.
! It is good practice to prescribe a target saturation range for all patients on admission so oxygen can be delivered if it becomes necessary, even if it is currently not needed.