The most common collective noun for a group of ravens is an unkindness. Other terms sometimes used are a conspiracy of ravens or a storytelling of ravens.
It is also appropriate to refer to a group of ravens by any term that can be used to describe a group of birds in general. For multiple birds together in the air, the proper word is a flight. A group on the ground or perched together is called a flock. To call a group of birds—especially birds kept together in a large cage or aviary—a volary is technically correct, but rare in modern English.