how many rooms are allowed on one lighting circuit?

Unless you do not have enough space in the service panel it is always a good idea to have each room on it's own circuit. If you want to put 2 rooms on a circuit just make sure those 2 rooms do not overload the circuit and wire it for a 20 amp circuit. Two bedrooms with 8 outlets each and 1 light each is about all you would want on the circuit. I would never put 3 rooms on the same circuit.