The ghosts will appear according to characters' expectations and are not necessarily a reflection of reality as much as the state of mind of the people looking into the storm. They may appear as people who died or might be dead, or even appear as friends who are still living, coming with some plausible story about how they did die thanks to the characters leaving them to come to Great Pines. It's basically preying on every potential doubt, producing images of people saying things they might not normally say, just to make the characters feel bad.

They can produce as many phantoms as they would feel guilty about running into. They absolutely have the potential of accidentally summoning an army with their guilt. :)