novemvirate vs octovirate

novemvirate

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

octovirate

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

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