A ream is a unit of paper, usually 500 sheets. While we have collective nouns like "flock of birds" or "herd of cows," there isn't a commonly used term for a collection of reams.
You could, however, use a descriptive phrase like:
* Stack of reams
* Pile of reams
* Bundle of reams
These phrases are clear and accurate, even if they don't have the same poetic ring as a traditional collective noun.