Legality of rent-by-room in NYC

[Michael Maliszesky]

I live in queens and I'm trying to get started in the landlording business. 

The cash flow numbers aren't very good in my area, but I'm want to look into the possibility of renting per room, possibility near a college.  For example, I could buy a 3 bedroom place near a college and rent each room to 3 different people. 

Does anyone know if this is legal in NYC? Do I have to register as a business or anything like that? Is there a limit to how many people can be in the house?