specs for LED driver to power 18 x XML T6
I assume the LEDs are wired in series.  This means you will need a boost driver with an output of about 70V. 

It depends on how much current.  The max current will be determined by the thermal management. These LEDs get very hot. Without an additional heatsink I estimate this board will handle about 250mA which will take the PCB temperature to about 80°C.  Too keep the temperature down to where it will not burn skin (50°C) you'd have to reduce the current to under 100mA.

If you were to use a Li-po battery pack with a 3.6V output you'd have an easier time finding a boost driver. 

An good alternative is to break the LEDs in to smaller strings and use a multiple output buck step down driver. e.g. three drivers each with 6 outputs like TI's LP8552 High-Efficiency LED Driver.