as a noun, 'plenty' refers to a large or sufficient amount or quantity. It is often followed by 'of' and then the thing that is in abundance.
There's plenty of time to finish the project.
We have plenty of food for the party.
'Plenty' as a noun is often interchangeable with 'a lot' or 'a large amount.'
as an adjective, 'plenty' describes having or being more than enough in quantity, degree, or size.
The room is plenty big for the two of us.
The soup is plenty hot, so be careful.
when used as an adjective, 'plenty' is less common than its use as a pronoun or noun. It's often used in informal contexts to emphasize that there's more than enough of something.
as a pronoun, 'plenty' is used to refer to a large or sufficient amount or quantity without specifying what exactly it is. It stands in for the thing that is abundant.
I have plenty, so don't worry about me.
You don't need to bring dessert; we have plenty.
when used as a pronoun, 'plenty' can often be replaced with 'enough' in the sentence, though the nuance might slightly change.