How many of these UV lamps do I need for PCB fab?
If you use enough to provide even coverage of the PCB you can adjust the exposure time (use a stepped test exposure) to get proper exposure for the size of blank you want to be able to handle.

I suggest you try visible lamps in the fixtures and adjust spacing and distance until the light looks even. Probably the horizontal centre spacing will be around twice the lamp diameter or less (right down to cheek-to-jowl) and the spacing from the board more than that. More lamps means faster exposure time, of course.

Do a google search for images of uv exposure boxes used in the screen printing field and you'll find a fairly wide range that works, but you want to err on the side of greater uniformity for PCB exposure.