Q:

Which chapter of the manga does Season 2 of One Punch Man anime end in? 

If you want to keep following One Punch Man story in the manga, after second season of the anime ended, you need to know which chapter of the manga the anime ends. 
Which chapter of the manga does Season 2 of One Punch Man anime end in? 

A:

As stated here, Episode 24 of the 2nd season of One Punch Man is based on Chapters 83-84 of the manga.  If you want to pick up where the anime ended, either you start reading on these chapters or proceed to the one that follows, which is Chapter 85.