Bunk Rooms - are they legal?

[Robert Saulnier]

Considering investing in property where I would like to get more "heads in beds" by adding a "bunk room" in the basement. This is based off a house on 4 bedroom septic which has 4 legal bed rooms. There is a finished basement and bonus room above the garage, I've been to many rentals that advertise 4 beds but sleeps 16, you go down in the basement and there are like 14 beds down there. What is the legality of this? Where is the risk not worth taking when adding beds to certain areas/rooms? 

I can't find any documentation online that really tackles this question. Trying to do everything by the book is pretty difficult but I also don't want to put any investments at risk.