Copies of Armored Core VI: Fires of Rubicon have shown up in the wild over in Japan, and those boxes appear to suggest that multiplayer will be included in the upcoming FromSoftware title.
The box shows that the game has online multiplayer for up to six players and also mentions that PlayStation Plus will be required for the title. The same box also suggests that up to three other players can spectate on whatever the multiplayer mode is. FromSoftware hasn't commented on whether or not Armored Core VI: Fires of Rubicon has multiplayer at all, but with the game launching next month they'll likely reveal more and more from the title.
FromSoftware titles aren't strangers to multiplayer, though they're never fully implemented. For example, the Dark Souls series uses multiplayer so that people can 'invade' and either help or hinder opponents in the game. Elden Ring, FromSoftware's most successful title, also uses this system in order to allow a PvP scenario to take place, and also features a full PvP arena specifically for multiplayer gameplay.
Armored Core VI: Fires of Rubicon is currently scheduled to launch on August 25 on PC, PlayStation 4, PlayStation 5, Xbox One, Xbox Series X, and Xbox Series S.