LED's are current devices, the "wattage" rating that manufacturers give is only an approximation. Meanwell Drivers power the LED strings based on teh forward voltage of the string, and set current rating (the current is what sets the LED's "brightnes").

Cree XP led's run at about 3.2-3.5 volts each, so a string of 12 LED's will run at about 38.4-42 Volts. A Meanwell ELN-60-48D (or P) driver can power between 24-48 Volts so your maximum number of LED's per string is around 12-13 (approximately).

Once you have your voltage calculations for your driver you then have to dial in your mA on your string to make sure the LED's are running within their rated spec's. For a Cree XP-E Series LED the MAXIMUM mA you should run them at is 1000mA, however a more conservative 750mA is better (they burn almost as bright, but run cooler).

You should definately do some additional research about your LED's before beginning your project. Using the standard meanwell ELN-60-48D (or P series) drivers you will need a minimum of 12 Drivers (You can run up to 13 LED's per driver).