heptarchy vs vigintivirate

heptarchy

noun
  • A government of seven people. 

  • The realm so ruled. 

  • A group of seven states, especially (historical) those in Anglo-Saxon Britain. 

vigintivirate

noun
  • A group of twenty people, especially (politics) a council of twenty men sharing an office or rule and particularly (historical) such an administrative council in ancient Rome. 

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