Rick was indeed provided some information when he arrived. He knows that there are at least twelve to fourteen possible victims. Possible, as it's hard to tell for sure who are victims of this dark entity and who are merely suffering from medical conditions if someone else wasn't there to see the dark spirit when the person collapsed. Of the fourteen, three have passed, and the rest are in comas. A commonality among them all is that all of them could be considered weaker or more frail than others, some with health conditions, some just very young or elderly. It seems this entity is targeting people who might be "weaker" in spirit, someone it can rip the soul out of with not as much resistance as a normal, healthy person might give it. It would explain why Sam's ghost friend was targeted, as Noemi was a spirit not even anchored to a body, just an item.

As for radius, this is something Rick isn't entirely certain on yet, but it seems to be within city limits so far. Timeline is also a little sketchy, as all Rick was told was "probably within the last month or so", which means some others in comas might also be victims and no one knows it.