well more phases if they are being interleaved would result in lower ripple current, leading to less ripple. That is the point of more phases in terms of performance, but in terms of a SMPS, more phases=less time each phase is on, this is only if each phase is one at one time, and not 2 or 4 on at one time. But then more components being switched on in general=less efficiency, so more phases would = less efficiency b/c of component number might out weigh the benefit of keeping FETs on for a shorter period of time, of course some arrangement's have 3-4 FETs per phase which would change this all together. But then you have technology that can just use as many phases as needed. Oh and there is always that thing with more phase sbeing able to output more power, but at a certain point you don't need so much power, but if a FET is outputting less then that is good too.
8 phases is enough for SB, SBe eats power tho.