Splatoon Raiders’ cooperative mode allows four players. The game supports cooperative treasure-hunting expeditions for up to four people, either locally or online.
That limit fits Raiders’ design as a single-player-focused action shooter with an optional cooperative layer. A solo player explores the Spirhalite Islands and fights waves of Salmonids, while additional players can join the same kind of expedition and contribute their own weapons, gadgets, and ink-based attacks.
The number can be confused with the larger team sizes used in Splatoon’s competitive modes. Traditional Turf War matches are built around two teams of four, which makes eight a familiar Splatoon number. Raiders’ cooperative mode, however, counts the whole participating party rather than two opposing teams.
Co-op also does not turn Raiders into a conventional competitive match. Players work together while searching for treasure and overcoming enemies, so the relevant answer is the total group size: four players, not four per side.