You can use as many PSUs as you want as long as you don't have multiple PSUs powering the same individual board (of which there is 9).
Do all PCI ports need to be powered? Or will it run with 2 on each board?
2 is fine, still only about 165W per PCI-E.
Are you certain that using 2 is fine? The product description page says all three are required. It may be that each PCIe plug powers a specific set of chips; the 3 PCIe connectors may not share a common power bus at all as in, say, the S3 design. Without knowing the actual S5+ design, or having an actual unit to look at, saying "2 is fine" is only speculation at this point unless something more is known than what bitmain has disclosed so far.
Bitmain could confirm this here, but they aren't posting much these days. If anyone already has a unit, they could also confirm whether using 2 out of the 3 connectors works and post here.
What Bitmain customer Service told us was we have to use all 3 connectors for each board, As my understanding if each board need at least 382W(3436/9), in average each connector should be bear
127w@10.6A , and if it works using only 2 connectors instead that will be
191w@15.9A for each connector , it is high risk to heat up or burn your cables if they are not good enough.
This still doesn't answer the question of whether all 3 are *required* by design, or can 2 be used if the cables can handle the increased current?
BTW the power requirements aren't as high as you mention: 3436W quoted by bitmaintech is at the wall, using a 93% efficient PSU. So the DC power to the S5+ would only be 3436*0.93 or 3196 watts, or 355W per board. *If* the design allowed the use of 2 connectors per board, this would put 178W per connector (14.8A). This isn't bad, considering S3+ units can easily be powered with 2 PCIe cables and those use around 355W as well.