How many lighting circuits are needed in the LED era...? (US - NEC)
As far as the NEC is concerned, there is no limit to the number of outlets (lighting outlets in this case) that are allowed on a single circuit assuming that the attached fixtures don't exceed the rated capacity of the circuit.

However, @Edwin raises exactly the correct point in the comment - what matters is what the fixtures are rated for, not what they are currently being used for. The general rule of thumb that electrical inspectors tend to use is around 8 boxes for a 15A circuit and 10-12 for a 20A circuit.  Keep in mind that they are counting boxes, not what gets put into them.  The idea is that you can put in a receptical rated at 5W, decide you don't like it, and throw something with a 150W load in the same receptical.  The only thing that makes something a "light" in the eye of the inspector is that it is on the ceiling and has a switch running to it.

Outside of the load consideration you actually bring up a good point in the question, which is safety.  While the NEC doesn't specify a minimum number of lighted branches (other than the panel lighting requirement), your local authority very well could.  I know my jurisdiction only allows 2 rooms to share the same branch for lighting purposes, precisely for the reason you mention - you pop a breaker, you're in the dark.