Consider the following:
This can be used as guideline for what corals will actual benefit from in terms of light spectrum. You can see below 400nm corals use very little if any. If you already use royal blues that peak at around 450nm then you probably have the lower spectrum pretty much covered. You could add a few more LEDs in the 425nm peak range, which I wouldn't consider UV leds but rather violet LEDs, just to be sure you've covered that lower section but how many is hard to say without knowing what you're currently using for royal blue. Note that purchasing true UV leds with ultra low wavelength of below 400nm is probably not beneficial to coral growth.
Here's another useful pic
Note if you can use LEDs that peak at 425nm in 50/50 relation with blue LEDs that peak at 475nm the use of royal blue becomes less important.