A vampire coven is a group of vampires acting together. These groups are formed out of friendship or out of survival, sometimes out of a collective goal, like revenge. Usually, vampires in the same coven hunt together, so it is very rare for them to have different diets (vegetarian or human diet).
A vampire coven, as commented by Edward in Eclipse, can be called "unusually large" if it has three members, thus covens with more then three members are enormous, and even a possible threat to the Volturi. Most vampires wander alone or with their mate. If a coven has more then four members, the Volturi will not take any action to protect them from possible attacks and will not regret their reducal of numbers, as seen in Eclipse.
Possible reasons for formingEdit
This type is formed when a group of vampires adopt eachother as their family, like the Olympic Coven. It is also when the coven exists of two vampires being eachother's mates. This type can interact with humans easily if they're vegetarians, because they seem like a normal family.
This type exists mainly in wars and is created as a group of capable fighters, ready to attack other covens, like the one Jasper was in. The Volturi can also be seen as a battle coven, as they have so much power over other vampires, they are feared by the vampire world.