Crisis on Multiple Earths, Volume 4

Strongly recommended, covering three late-silver age crossovers between the Justice League and the Justice Society. Earths covered: Earth-1 and Earth-2, of course. Also Earth-Prime, Earth-S, and Earth-247. Sort of.

If this is the length that the volumes in this series is going to shoot for, approximately, then Volume 5 would end up covering Earth-898 (with the western heroes crossover), as well as the murder of Mr. Terrific and the New Gods team up.

Volume 6 would then get the Secret Society of Super Villians rematch and the big 5-issue crossover with All Star Squadron.

Leaving not much for and increasingly hypothetical Volume 7: 'Family Crisis', scribed by a young Kurt Busiek, and a largely incomprehensible crossover with Infinity Inc. during the height of the original Crisis on Infinite Earths.

Getting to V. 6 soon would be nice. But going on to V. 7...not so much a priority, I don't think. Especially with lots of excellent stories to go into the companion "Team-Ups" collection series. (Like the DC Comics Presents Annuals, for one.)

UPDATE: On the other hand, the two issues in the Legion crossover in V4 were both giants, unlike any of the ones in the future. So you might be able to squeeze the SSSV story's 3 issues into a Volume 5, leaving all of the remaining issues for Volume 6...