quantifiers - "Four dozen" or "Four dozens"

Out of the three, which is the best option and with which grammatical rule:

1. I have four dozens of mango.


2. I have four dozen of mangoes.


3. I have four dozens of mangoes.

Answer

Hundred, thousand, pair, dozen, and couple take the same form both in the singular and in the plural only when they are used after numerals or after several or a few.

Three hundred (not hundreds), two pair, three couple, five dozen etc.

Example:

• He gave me a sum of three hundred rupees.
  


• He bought two pair of shoes.
  


• There are five dozen mangoes in the basket.

NOTE: In modern English, "pairs" and "couples" are more usual.

Example:

• He bought two pairs of shoes.
  


• Five couples were present at the meeting.
  

When the nouns of numbers are followed immediately by "of," their plural forms are used.

Example:

• Hundreds of people.
  


• Hundreds of thousands of people.
  

Based on that, the correct answer is #2:

• I have four dozen mangoes. or I have four dozen of mangoes.


• I have two pair of dresses.
  

But, we say:

• I have dozens of mangoes.
  


• I have a number of pairs of dresses.

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EDIT

Since many are concluding that using "of" is not correct, It is uncommon, but it is not wrong either, maybe to American/British, but It is very common to use "of" after "dozen" in India. Since the OP is Indian, I have provided a source from an Indian book.

But as you can see here, the results are very contrasting to the previous one:

Here is something that may help you: English Grammar without Tears