a lot vs a number of

a lot

pron

  • A large amount. 

  • Many things, much. 

adv

  • Very much; a great deal; to a large extent. 

  • Often; frequently. 

a number of

phrase

  • Several. 

  • Several of. 

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