I have read that dozen, hundred, thousand etc. have no -s and of when they are used after a numerical, a few or several : Three dozen eggs. A few hundred times, Several thousand years. But with some and many, they take -s, and of is used : Some/many dozens of mangoes.
THERE ARE SOME SIMILAR TYPES OF QUESTIONS IN THIS FORUM, BUT THEY DON'T CLARIFY THE FOLLOWING (PARTICULARLY MY 2ND QUESTION) GIVEN BELOW : 1. "Three dozen eggs" is correct; but "three dozen of eggs" is not correct. Why? 2. "Some/many dozens of eggs" is correct; but "A few/several dozens of eggs" is incorrect. Why? I am really confused. Could anyone of you please clarify my queries with brief & precise grammatical explanation.