Power Supply Selection for PWM Controled Landscape Lighting
A quick and dirty method is to add the wattage of all the LEDs you plan to use, and double this. The find a power supply with the power and voltage meeting your spec. 

Also consider future expansion - if you think you might add additional zones in the future, add those to the total now, so you don't run into a problem later.

Also over design your wire gauges by at least 100%, that way you can safely tap a new zone off of an existing one (within reason). 

Also - I honestly think your design will be simpler if you have just a single supply, with a separate dimmable (PWM) circuit. Because you're using PWM to dim your lights, it uses less power as the current is switched off for a period of time. But you have to factor in the possibility that all lights are on full brightness.