a great many vs plenty

a great many

det

  • A substantial number of 

plenty

det

  • many 

  • much, enough 

noun

  • A more-than-adequate amount. 

adv

  • More than sufficiently. 

  • Used as an intensifier, very. 

pron

  • More than enough. 

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