"The revelation, contained in an updated page about the 2013 hack, is the result of new information and the forensic analysis of an unnamed security consultant. Previously, Yahoo officials said about one billion accounts were compromised. With Yahoo maintaining roughly three billion accounts at the time, the 2013 hack would be among the biggest ever reported."
https://arstechnica.com/information-technology/2017/10/yahoo-says-all-3-billion-accounts-were-compromised-in-2013-hack/
https://arstechnica.com/information-technology/2017/10/yahoo-says-all-3-billion-accounts-were-compromised-in-2013-hack/