5 lights.

1 Infinite for sunlight from outside.

2 point lights (blue) for the big window and the skylight. On these I put a position-dependent shader so that the light intensity fell off with the distance from the window or skylight to the power 1.3.  (For point lights, the intensity falls off with the distance squared, or power 2. An infinite sheet light falls off linearly, or power 1. A bounded sheet [such as a window] falls off somewhere inbetween.)

1 point light (yellow) for the lamp in the corner

1 IBL (no image at all - just a constant ambience) to simulate secondary bounced light within the room.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)