quinquevirate vs septemvirate

quinquevirate

noun
  • An official group of five people, especially a council of five men sharing office or rule. 

septemvirate

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

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