Two volumes have been announced so far, with Volume 1 (ISBN 1421590700) due for release on September 6, 2016. Volume 2 (ISBN 1421590166) is scheduled to follow on December 6, 2016. (Pokémon XY volume 8 will be released in the middle, on October 4).
As Volume 1 is 96 pages in length and Volume 2 is 112 pages in length, it is assumed that these releases will be "mini-volume" length, comparable to VIZ Media's previous Pokémon Black and White and Pokémon XY releases. Comparatively, the first Japanese volume collecting the Omega Ruby & Alpha Sapphire chapter is 194 pages.