The driver isn’t regulated, so the cell voltage will be the same voltage that goes over the LED.
According to this Sofirn SP40: Headlamp with some hidden features it will take a single CR123A.
It might depend on the LED though, as the lower voltage of a CR123A may not be over the forward voltage of some LEDs.
I wouldn’t recommend 2 cells on series unless a flashlight specifically recommends it.