Is army common noun

Yes, the noun 'army' is a common noun; a general Word for an organized military force equipped for fighting on land; a large number of people organized for a particular purpose; a general Word for any army of any kind.

A common noun is capitalized only when it is the first Word in a sentence.

The noun 'army' is a standard collective noun for:

• an army of ants
• an army of caterpillars
• an army of frogs
• an army of herring
• an army of soldiers