You can calculate the input voltage that you will need using the following formula:
Vt + (Vf x LEDn) = Vin 

Where:  
Vt  = 
2 if you are using a DC BuckPuck or PowerPuck driver 

Vf  =  The maximum forward voltage rating of the LEDs you are powering  
LEDn  =  The number of LEDs you want to power  
Vin  =  The input voltage to the driver 
For example, if you need to power 6 LXMLPWC2 LEDs from a DC power source, then Vin would need to be at least 22 VDC based on the following calculation:
2 + (3.25 x 6) = 21.5
The Vf rating of an LED provided in the technical documentation is the typical rating. For example, the typical Vf for the LXMLPWC2 is 2.9. To ensure that your input voltage is high enough to power the LEDs if all of the LEDs you are powering happen to have the maximum Vf rating, you should always use the maximum Vf rating of the LED in your calculation. In the case of the LXMLPWC2, this would be 3.25.
If you are using an AC BuckPuck driver, then be sure to use 4 in your formula.
This formula determines the minimum input voltage you need to provide to the driver. There is no harm in using a higher voltage up to the maximum input voltage rating of the driver. (32V for DC drivers and 28V for AC drivers.)