What is a group of foxes called?

There isn't a formally recognized collective noun for a group of foxes. While "leash" of foxes is sometimes used, it's not widely accepted and feels less natural than other collective nouns (like a murder of crows). The most common and accepted way to refer to a group of foxes is simply a group of foxes, or sometimes a pack of foxes, although a pack is more specifically associated with their social structure during hunting and raising young. Using "pack" might be appropriate when discussing a family unit, but for a more general collection of foxes, a "group" is the best choice.