1 how many receptacles to put in
2. would 4 recessed can lights be ok?
3. Should the inside walls and ceiling be painted black?
4. Anything else I should know?
Oh boy. Neat project. You'll make someone very happy.
I managed a wet darkroom facility about 40 years ago.
1) I'd suggest 4-5 receptacles, all with GFI outlets. (I've been shocked in darkrooms. Not Fun.)
2) Two light circuits might be best. One for the 'cans' which will do ordinary room light, and a second for a ceiling or high wall-mounted receptacle which will power the 'safe lights', special lights with filters that limit the light to colors that B&W print paper doesn't respond to. If he has the screw-in safelights with an Edison base, then maybe a couple of old school porcelain fixtures on the second light circuit.
3) Black paint everywhere is overkill. There is probably a bench or high table that the photo enlarger, a sort of projector used to make prints from negatives, is sitting on. A flat dark gray or black paint on the wall behind and ceiling over this, and on some sort of partition or screen on each side of it should be sufficient. The idea is to prevent reflections from the enlarger light from striking the print paper. The lab I ran had a row of enlargers, with the ceiling over the equipment, wall behind them, and partitions between them all painted flat black.
4) Make sure there is a good light seal around the door, including at the bottom.
For extra credit, put another porcelain fixture on the safe light circuit outside the door to the darkroom, with a big red bulb screwed in. When the safe lights are on, the photographer is likely to be doing things with light sensitive print paper. The big red light wars others not to open the door. (Film guys: We always used changing bags when opening film and loading it onto processing reels. It was a multi-person lab, and blacking out the room while one guy loaded film wasn't practical. Even when I had my own darkroom, I found fewer 'accidents' happened if I always used a bag when handling my precious exposed film.)