depends on how you want your tank's light distribution to work, what kind of coral you want to have, and how you want to design your tank
if you want to keep softies/LPS mostly, and want even distribution, then go with 4 - 6 primes
If you want to keep half the tank high light for SPS, and the other half low light for your LPS/Softies, then keep the 2 primes on 1 side, and get something with a lot more power on the other side.
If you want to have a true mixed reef with SPS/LPS/Softies all mixed together with SPS in the high light areas and lPS/softies in the bottom/in the shade, then you'll need to spread the high power LEDs and the primes mixed tgoether, or go with a T3/PED hibrid.
as a comparison - my 29G has 3 primes b/c i'm keeing acros.