at one time vs once

at one time

prep
  • formerly 

  • simultaneously, at once. 

once

adv
  • Multiplied by one: indicating that a number is multiplied by one. 

  • One and only one time. 

  • Formerly; during some period in the past. 

conj
  • As soon as; when; after. 

How often have the words at one time and once occurred in a corpus of books? (source: Google Ngram Viewer )