Take this with a grain of salt for now as nothing has been officially confirmed, but an image on Reddit has been shared that suggests competitive in Call of Duty: Black Ops 4 will be transitioning to 5v5 for the upcoming season.
Treyarch made public matches 5v5 across all modes, with a bonus “Chaos” mode allowing the typical 6v6 format.
A new image, from what appears to be the PC beta files for Black Ops 4, showcases that there are 5v5 players in the League Play settings for Black Ops 4. It shows 5 players on the back drop for each team.
Activision has not officially confirmed any details on the season format for Black Ops 4. An official announcement is expected on September 19 at 10AM PT during the esports reveal stream.