Alongside many other additions to Super Smash Bros. that we've seen in the latest Nintendo Direct, Nintendo is bringing back the tournament mode that was first popularized in Melee and was also present in Brawl, although it has been somewhat marginalized due to the introduction of the Adventure Mode. The maximum number of participants is 64, and perhaps the most interesting addition is the fact that you can watch replays of each fight after the tournament is complete.
Tournaments should not be mixed up with tourneys, another new feature that essentially allows you to create and host your own online tournaments.
There are no additional details on this mode. A screenshot in the direct shows a maximum number of participants set to 30, as well as the possibility to set different winning conditions and start / end dates. This suggests that it will be possible to create more than single-sitting tournaments in the classic Smash Bros. sense but actual long-lasting... well, tournaments.
Source: Super Smash Bros. Direct