Use single or 2 cores

[Vaer]

Personally I would not cheap out on it unless you are upgrading fairly regularly. (Think every year or two.) I would pay the extra/save up a bit more to get a slightly better one if you are going to be keeping it for a long time. The current AMD offerings are mostly subpar for gaming so it's not really recommended that much.

The min. spec is going up (they haven't released the entire thing yet) in Stormblood. Maybe wait a couple days for the Fanfest where they might release it. The old recommended spec is probably leaning around a quad core (Intel® Core™ i5 2.66GHz) with the old minimum spec at a really old dual core (Intel® Core™2 Duo@3GHz).

Wait a couple months for AMD Ryzen as there may be price shakeups depending on how good it is and the price/perf before buying if you can wait. Something along the lines of a 6500-6600/k is enough if you're only using it for XIV if you need something immediately. If it's too expensive, maybe a i3.6100.