You're question can't be answered because there is too little context.
These are buck converters. Below I will compare 4 vs 8, but it applies to 6 vs 12 or any other combination.
1) In general, more phases are better because they will produce less ripple. But, all buck converters produce ripple and the final result depends on componemt quality and filtering. So, 4 phases filtered well could have less ripple than 8 phases filtered poorly.
2) Companies lie about the number of phases. It is quite common to use doublers, use 4 phases, and call it 8 phase. It's simply a lie, but common. This makes the ripple assumption completely wrong. Doublers double the current capability, but don't halve the ripple as expected. They produce a signal which has only half the number of advertised phases. So, if you go by their advertising alone, you have no idea which one is truly the best option. Dig into the details.
3) The other reason you may want more phases is for higher current capability. Again, this is only an assumption, because 4 phases with better components could handle more current than 8 cheap phases.
TLDR: the advertised number of phases is marketing and means absolutely nothing on its own. If you want recommendations, give some product options to compare. You can only compare apples to apples, and if you provide no details, everyone is wasting their time guessing.
