When operating low voltage lights, you need a transformer to convert your standard line voltage (120V or 277V) into low voltage (12V or 24V). This allows your lights to function properly. If you connect low voltage lights directly to line power, the higher voltage would cause them to burn out immediately. The transformer makes them compatible, sort of like a USB adapter for your iPhone.
To figure out how many lights you can power with a single transformer, look to the minimum and maximum wattage ratings. The transformer's minimum wattage rating tells you the smallest number of watts it needs to power in order to work. The maximum wattage rating, as you probably guessed, indicates the largest total number of watts a transformer can handle.
There are two kinds of transformers you can use – electronic and magnetic. Electronic transformers are cooler, quieter, and more compact than magnetic transformers. But, electronic transformers can't power more than 300 watts, while magnetic ones can power as much as 1,200 watts.
When using an electronic transformer, the total wattage of the light fixtures connected to the transformer should be less than or equal to the maximum wattage rating. For most magnetic transformers, the total wattage of the lights you connect shouldn't be more than 80% of the maximum wattage rating. This is because the cheaper design of many magnetic transformers can cause additional power loss.
So, if you want to connect eight landscape lights to an electronic transformer, and the lights use 20 watts each, you would need a 160 watt transformer. (8 lights X 20 watts = 160 watts).
If you want to connect ten landscape lights to a magnetic transformer, and the lights use 30 watts each, you would need a 375 watt transformer. (10 lights X 30 watts = 300 watts, and 300 is 80% of 375).