In an Online Article, Hajime Isayama, the author of Shingeki no Kyojin (Attack on titan) has revealed the series to end in 20 volume. Here's the link to the article.
it's not clear if it's 20 more volume or if it's ending on volume 20. I'm going to assure the ending on volume 20 since it's a nice even number of chapters.
Since the volumes are roughly around 4 chapters each, and we're on volume 11, we can assume that there will be at least 36+ more chapters of attack on titan. Meaning the series will end somewhere around chapter 100+.