There doesn't appear to be a limit to the number of enemies that will be effected, although there is a distance limit. I would say the distance is roughly 3-5 "shoulder widths" of the standard human enemy. It's relatively similar to the radius of a trap created from that particular Vigor.
The effect is the same as the base Vigor effect, so if an enemy is immune to the effect, they will not be effected.
There doesn't appear to be any stacking bonus from having multiple effects of this type equipped simultaneously. Overkill + Storm was the combo I tested, and I didn't see any difference between the effects.
They will chain to non-hostile NPCs, and as jmfsg points out, there are times when you're in a mixed fight with armed allies and enemies - this can lead to a bad situation if you turn your allies hostile. In situations where there are "neutral" NPCs (ie, peaceful civilians) or charmed/friendly automatons, the only effect is damaging or destroying them.
There are three:
The first code book is in the Soldier's Field gift shop, in the cannon in the central area. This book decodes a code in the bathroom of the restaurant across the street. The decoded message is "Tip the hat to the Vox" and refers to a hat rack in the bathroom next to the one where you find the code.
Behind the hidden wall is a secret room with an Infusion, a Voxophone, and some assorted weapons including an RPG.
The second code book is in the Good Times Club lockup, in the room where there is a large fire and some books being burned. The code unlocks a safe hidden behind a clock in the Plaza of Zeal area. In order to get into this room, you'll have to pick a lock that requires 1 pick. The code reads "The broken clock tolls at midnight" and refers to a clock on the wall to the right of where Elizabeth stands when she reads the message.
Inside is an Infusion, a Voxophone, some lockpicks, and a volley gun.
The third code book is in the basement of Hudson's clothing store, a shop near the Bank of the Prophet. It unlocks a hidden room in the basement of the Bank of the Prophet. The translated code is "type three letters to spell the voice" and refers to a trick typewriter on the desk where you find the coded message.
Behind the shelf is another Voxophone, another Infusion, some lockpicks, and a couple of weapons.
IGN has video guides for each of these, and they also include guides to the two locked chests you'll encounter as well.
Best Answer
Due to lack of other in-game sources, and seeing that "only one obstacle" was written on a picture of the lighthouse, the "only one obstacle" that the answer you mentioned says (the lighthouse keeper) is the closest possible "obstacle". This is also since the lighthouse keeper maintains and guards the "entrance" to Columbia (the "chair" in the lighthouse). Booker won't be able to go to Columbia unless the Luteces find some way to deal with the lighthouse keeper.
There's a hint in the lighthouse that Comstock ordered the lighthouse keeper to stop Booker from getting to Columbia:
Click on image thumbnail below to view larger, original image.
In the answers to this question: What was up with the coin flip question? - we learned that the "Booker DeWitt" player character is the 123rd Booker DeWitt that attempted the rescue of Elizabeth. All other Bookers that the Luteces assisted in rescuing Elizabeth have failed for some reason. We also learned of constants and variables in the different multiverses. e.g. Booker flipping heads and not "rowing", is a constant, and whether Booker chooses a cage or a bird brooch is a variable.
Why the lighthouse supervisor was the "one last obstacle" will depend whether he is a constant or a variable.
Possible explanation on why the lighthouse keeper is the "only one obstacle" if he's a variable:
i.e. Booker will only get on the chair if the lighthouse keeper is killed.
Source: Comment at Reddit /r/Games subreddit post, "Lingering mysteries of Bioshock Infinite" by Reddit user, Hector_Kur
If the dead lighthouse keeper is a constant:
i.e. In all universes where the Luteces sent Booker to Columbia to rescue Elizabeth, the "only one obstacle" in getting Booker to Columbia is always the lighthouse keeper.
Source: Answer that OP mentioned on the "Who is the dead man in the lighthouse?" question, by @Answergaming