The British Society of Thoracic Imaging (BSTI) have published guidance (attached below) on grading severity in COVID-19 using CT into Mild, Moderate and Severe. Similar principles can be applied to establishing severity on Chest Xray.
Unifocal or minor bilateral focal/patchy infiltrates (<3)
More extensive patchy bilateral infiltrates (>3) or early limited peripheral consolidation
Extensive consolidation or confluent peripheral infiltrates