You will have 3 LEDs in series in each string. If those are Cree LEDs the voltage when they are on will be from about 3.1 to 3.6 volts per LED, or from 9.3 to 10.8 volts for each string. That means you can use a 12 VDC power source. Each string will draw from about .5 to 1 amp, so with 6 strings in parallel they will draw from 3 to 6 amps total current. The power source must be capable of supplying more current than that, because it doesn't work well running an inexpensive DC supply at its maximum current full time. Assuming you have the high powered LEDs, you would want the supply to be capable of at least 8 amps, in my opinion. However, a buckpuck between the power supply and the LEDs means the LED current isn't exactly what the power source supplies. That might make it prudent to have a bit more current capability, or possible to have less.
12 VDC power supplies are cheap for low current, so you could save money using multiple power supplies, each powering a few parallel stings.