octovirate vs undecimvirate

octovirate

noun
  • A group of eight people, especially (politics) a council of eight 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 octovirate and undecimvirate occurred in a corpus of books? (source: Google Ngram Viewer )