How many pairs of wire to use for Power over Ethernet?
It depends on the total resistance the line will have. You may not transmit sufficient power at that voltage, that is where that easy solution is highly considerable; doubling the wires, halfing the resistance :-) If they are not in use for any other thing, I recommend to use them anyway.

It is about its yet unknown power requirements of -after regulator- circuits and design of regulators it have. But simply, observe the device if it has tolerance or not for that amount of resistance given. I mean if everything are OK on all circumstances, different modes etc. I said use the all pairs anyway, because you may not encounter all the power states it will have at that time. So, briefly; if it works with single pair, use two pairs and you are fine. If it works fine only after using two pairs, observe it a longer time. If there is failure, than you have to calculate and make the voltage sufficiently higher on the line and use a buck converter for nominal voltage before the load.

Somebody might say it wouldn't be that much problem for 10-15 m cable; the resistance of ethernet cables are not standart. There has been cables I had seen even as high as 1 Ohm/meter.