You can calculate the number of seriesconnected LEDs you can power with a single BuckPuck or PowerPuck driver using the following formula:
(Vin  Vt) / Vf = ↓LEDn 

Where:  
Vin  =  The input voltage to the driver  
Vt  = 
2 if you are using a DC BuckPuck or PowerPuck driver 

Vf  =  Maximum forward voltage rating of the LEDs you are powering.  
LEDn  =  Number of series connected LEDs you can power (results rounded down to the nearest integer) 
For example, if you have a 12VDC power supply, the maximum number of LXMLPWC2 LEDs that you could power in series with a single driver would be 2 based on the following calculation:
(12  2) / 3.25 = 3
Remember that 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 for Vt in your formula.