Multiplayer first person shooter games with 6-player online support

Starship Troopers: Extermination

Players: 16 local, 0 online

Call of Duty: Ghosts

Players: 6 local, 0 online

Duke Nukem 3D Megaton Edition

Players: 8 local, 0 online

Duke Nukem 3D: 20th Anniversary Edition World Tour

Players: 8 local, 0 online

Call of Duty: Ghosts

Players: 6 local, 0 online

Serious Sam: The First Encounter

Players: 16 local, 0 online

Killing Floor 2

Players: 6 local, 0 online

Killing Floor 2

Players: 6 local, 0 online

Insurgency: Sandstorm

Players: 8 local, 0 online

Unfortunate Spacemen

Players: 10 local, 0 online

Serious Sam 2

Players: 6 local, 0 online

Killing Floor 2

Players: 6 local, 0 online

Quake

Players: 16 local, 0 online

Call of Duty: Ghosts

Players: 6 local, 0 online

Serious Sam: The Second Encounter

Players: 16 local, 0 online

Call of Duty: Black Ops

Players: 6 local, 0 online

Contagion

Players: 8 local, 0 online

Killing Floor

Players: 6 local, 0 online

Call of Duty: Black Ops

Players: 6 local, 360 online

Synergy (HL2 mod)

Players: 16 local, 0 online

Starship Troopers

Players: 8 local, 0 online

Team Fortress 2

Players: 6 local, 0 online

DOOM 3: Last Man Standing (Mod)

Players: 16 local, 0 online

Duke Nukem 3D

Players: 8 local, 360 online

Mission Against Terror

Players: 8 local, 0 online