128Gb. Since there's a need for at least 64Gb in areas, plus the needs of the pc to actually function, above and beyond that 64Gb, 128Gb is the next dual channel size. You could mix and match to get 96Gb, but that would end up with 64Gb in dual channel and 32Gb in single channel, which could offer downgraded cpu performance.
UE5 is almost virgin territory, there isn't much on it yet, but if it has the ram use possibility, skimping on ram is the last thing you'll want to do. Of course the cpu/platform will also have a lot to say in the matter, there are many platforms that simply do not like more than 64Gb.