novemvirate vs undecimvirate

novemvirate

noun
  • A group of nine people, especially (politics) a council of nine men who share office or rule. 

undecimvirate

noun
  • A group of eleven people, especially (politics) a council of eleven men sharing office or power and particularly (historical) the groups of eleven magistrates in ancient Athens. 

How often have the words novemvirate and undecimvirate occurred in a corpus of books? (source: Google Ngram Viewer )