is essentially a noun, and is used either by itself or with of + following noun (plural, or singular mass noun): We have plenty / You will find plenty of books / There is plenty of time. Use of plenty as an adjective without of is found in regional forms of English but is not standard:

• Although there are plenty other ideals that I should prefer —Robert Louis Stevenson

• Leopard Society in Sierra Leone. They kill plenty people —Graham Greene, 1969.

Use of plenty as an adverb meaning ‘very, clearly, more than usually’ is restricted to non-standard AmE:

• He seems plenty dead to me —R. Silverberg, 1985

• I frowned at my mother plenty —New Yorker, 1990.

Modern English usage. 2014.