Answer:
The word plenty is a noun, an adverb, but after those two designations, the seven dictionaries (on line and hard copy) that I consulted give conflicting information. Some do give 'plenty' the designation as a pronoun; some also say that 'plenty' is an adjective. Let's start with the clearly defined uses:
Noun: We have plenty of milk but we're out of cereal.
Adverb: Be sure to bake plenty of cookies for the party.
Now, one dictionary says that my use of plenty as an adverb is an adjective describing cookies, however, a similar sentence was worded, 'bake plenty cookies', not a use that I'm familiar with without the 'of'.
I tend to agree with the dictionaries that designate 'plenty' as a pronoun, an indefinite pronoun. Here are some examples:
Plenty of the people like chocolate.
There are plenty of people named John.
There are some gifts but plenty of envelopes of money.
Although the use of 'plenty of milk' and 'plenty of people' are extremely close in context, 'plenty of milk' is a sufficient quantity, a known quantity; 'plenty of people' is an unknown quantity.