These phonebook-sized, B&W, value-priced collections each contain about 25 issues worth of X-Factor volume 1 continuity.
Due to crossovers and initial set-up, this title has a lot more tie-in issues from other books included in order to grant full stories, so while most titles with Essentials treatment would be close to issue #75 by the third essentials volume, X-Factor is only up to collecting Issue 50. If a 4th volume gets printed, it should finish out the run of the title before it was drastically altered / relaunched with Peter David's run.
— Description from Comic Vine community (comicvine.gamespot.com · https://comicvine.gamespot.com/essential-x-factor/4050-24745/).
