dozen和hundred的用法相同。

不定冠词或数词+dozen/hundred +复数名词。 例如 a dozen books, two dozen books, three hundred people...

dozens/hundreds of +复数名词。例如，dozens of books,  hundreds of people,  dozens of thousands of people ...

所以，a dozen of books, two dozens of books, dozens books, two hundreds of people, hundreds people... 均为错误。