Fun is an adjective (sometimes an adverb), a word or phrase describing a noun or proper noun (or a verb in the adverb case). Therefore it is neither plural nor singular. It only describes nouns.

“That was fun.” Fun describes that. (adjective)

“Writing is fun.” Fun describes writing. (adverb)

“Writing is a fun activity.” Fun describes activity. (adjective)

No one who speaks standard English says “We will have funs.”