Yahoo claims to have 1 billion active monthly users right now. That's different than total users, and that number could be much higher than it was in 2013. Either way, with two separate hacks in 2013 and 2014 impacting between 1 billion and 1.5 billion accounts, it's a good bet that yours was among them.

http://www.forbes.com/sites/leemathews/2016/12/16/what-to-do-if-you-think-your-yahoo-account-was-hacked/#f175db930def