What is the maximum power transmitted through NFC that can be harvested?
What is the maximum power transmitted through NFC that can be
  harvested?


The maximum power that can be transmitted wirelessly is limited only by human imagination and the cost of Litz wire: - 



Here's an example using copper tube coils (litz wire would be better of course): -



It is a highly resonant magnetic field antenna (a coil) with probably many tens of amps flowing in the primary. However, the problems faced in transmitting tens of watts at over a metre distance is huge and uneconomical compared to wires. At the sort of distance shown, the receive coil will be seeing a fraction of the B field originally generated by the transmission coil but, by parallel resonant tuning the receive coil (as is done at the transmit coil) you can extract a few watts at several inches.

So, when this is all scaled down to NFC devices, you have to ask the question, is the NFC card reader capable of producing fields that are capable of sustaining several watts in the NF tag/device. I doubt it very much because the a payment card (for instance) requires only milli watts to activate its electronics and why would the generating device want to be able to provide anything more than a few tens of milli watts? 

Here's an example of the technology: -



The receiver/harvester is the MAX66242 and its data sheet states that the device needs about 2 mA at about 3 volts, that's a power of 6 mW. It also states that the minimum field required for operation is 0.15 A/m and the maximum field is 5 A/m so, if 6 mW can be extracted from a field of 0.15 A/m then maybe 40 times this power is extractable from some transmitting devices capable of generating 5 A/m.

So maybe a couple of hundred mW could be extracted.