According to Oxford Dictionaries, a group of nuns is known as a superfluity. Although the term is now rarely used to refer to nuns, it is sometimes used to refer to an excessive amount of something.
The New Collegiate dictionary states that the word superfluity has a 14th century origin. The linguistic source of the word can be found in the Latin term superfluus, the Anglo-French word superfluité and the Middle-English word superfluitee. A single nun, a woman who has made solemn vows to a religious order, is generally called a sister, though some religious orders also refer to her as mother or dame.