User blog comment:Masterzach32/Item discussion - Black Cleaver/@comment-7481207-20140430200100

A quote from the notes on The Black Cleaver's wikia page:

"The armor reduction passive stacks with itself, purchasing two The Black Cleavers will cause physical attacks to apply two stacks per hit, however the maximum 5 stacks still remains."

Meaning that regardless of how many black cleavers you have, the maixmum armor reduction is 25%, you only get to that point faster if you have more cleavers.