While "a brood of trolls" or "a pack of trolls" could be used, they aren't as established as other collective nouns like "a flock of birds" or "a herd of elephants".
You could also use more descriptive phrases like:
* A horde of trolls
* A coven of trolls
* A company of trolls
* A gathering of trolls
Ultimately, the best collective noun for trolls will depend on the context and the specific group of trolls you're describing.