I could only find until volume 12 and I read somewhere that volume 13 and 14 will be the conclusion. I would like to know if it's already released. Please help
Gagaga Bunko, the publisher of Oregairu light novel made a tweet on 28 Sept 2017 (Japanese):
To put it simply, volume 12 is the 1st part of the 3-part final chapter, and volume 13 & 14 ―the 2nd and 3rd part― will be published almost at the same time.
However, as of current writing, volumes 13 & 14 haven't been published.