If you’re counting the number of rooms in an NYC apartment, the rule is simple: a separate kitchen is a room. A bathroom is not.
Thus a “two-room studio” is a room where you would live and sleep, attached to a room with a kitchen in it.
A “one-room” studio (which realtors would just call a “gigantic studio” or “cozy studio,” depending on size) is a room where you would live and sleep with a kitchen built along one wall (that’s called a Pullman).
Also, “bedrooms” in NYC-real estate agent speak have windows.
Thus an apartment with a bedroom, living room, and kitchen is a three-room apartment; a room with a bedroom, living room, kitchen and non-windowed alcove is a one-bedroom, 3.5-room apartment; and a room with a bedroom, living room, kitchen and windowed alcove, while it is also a one-bedroom, 3.5-room apartment is advertised as a “junior four” or a “convertible two-bedroom” because you could wall off the dining alcove and make a fourth room and stick a baby in there, if you chose to eat standing up in the kitchen for the rest of your life.
These definitions seems to be merely terms of art — I spent a morning reading the NYC building code and found the definition of a “chimney” but not a “bedroom,” but they’re pretty widely used.
Also a “Classic” is a big pre-war apartment with little tiny bedrooms for the servants. So a “Classic Seven” is a living room, dining room, kitchen, three adult bedrooms, and a maid’s room.
The left-hand apartment (in green) in the illustration above is a Classic Seven. The four-room apartment to its right would be marketed as a two-bedroom. Image of the Dartmouth courtesy of the New York Public Library.