Electrical – Extremely low suspended ceilings? (NEC)

You could try something like a Philips SpotOn. LED light, batteries-operated, motion-sensor-activated. Ideal for small, dark closets and cabinets.

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.