Is plethora an adverb or adjective?

Answer 1

It is neither an adjective nor an adverb. "Plethora" is a noun.

Use it like you might use the nouns "abundance" "quantity" or "excess", when what you want to convey is "huge amount" or "overabundance".

I don't think it is often pluralized; have never seen that, anyway.

Also rare would be its adjective form "plethoric", leading to a stretch to make the adverb "plethorically".

Good for "plethora" for being one of those words that has very very few relatives in the language!