The plural form of severity is severities.
Can major be plural?
The plural form of major; more than one (kind of) major.
Can adjectives be plural?
Adjectives don't have a singular and plural form OR a masculine, feminine and neutral form. Adjectives are always the same! Never add a final –s to an adjective. Adjectives can also be placed at the end of a sentence if they describe the subject of a sentence.