For the most part, it goes like this:
Two dozen eggs; two dozen cats; two dozen books; two dozen bottles of beer; etc.
Two dozen of the ones in the back; two dozen of these pink ones, two dozen of those my sister told me about; two dozen of the 400 students; etc.
In other words, only use of if you are talking about two dozen (or any other amount) of a specific larger group.
I hope this is helpful,