User blog comment:Honey Pie/In the end, will the anime have 3 Seasons?/@comment-26095689-20151103123958

It will probably be around 27 volumes. In an interview at that time Vol 17 being published. He said it was 60% done.

http://www.crunchyroll.com/anime-news/2015/09/05/editor-discusses-beginning-and-end-of-attack-on-titan

https://antenna.jp/news/detail/1712153/