Its probably fun to play with 27 players in co-op, but if you want to keep it real, I would probably say 9 players. No more no less. The ghosts are not an army, they are a spec-force of some sort.
If I where the developer I would probably have 9 players in co-op and leave the 27-player-thing to the modders. 9 AI-soldiers in SP would be a bit difficult however, since you only play as one of them. So I can understand why the devs decided to only have 4 players in co-op, to match the SP, right?