I know nothing about the ZW or Legacy, so someone else will have to chime in. My best guess it that the ZW will be just fine running 2 engines with 1-2 more sitting on the sidings.
The key to how many trains any transformer can support is amps. Each train draws amps and that depends on the engine, how much weight is being pulled, etc. I have a small 100 watt MTH Z-1000 that provides ~5.5 amps. It runs 2 trains (steamer with lighted passenger cars and a diesel) with a single connection to an 8x14 Christmas layout on MTH RealTrax. My understanding is the ZW outputs ~16 amps at full power, but realistically ~11 amps under normal operation. That is just something I read after a Google search. Bottom line is amps are important when determining how many trains can be run without adding more power.
There are different kinds of blocks. Power blocks used to be needed in order to run multiple trains and I guess still are for conventional running. Modern trains use digital controls where fixed power is run to the tracks and systems inside the engines determine how much power goes to the wheels. That way you can run multiple engines on the same track without the need for blocks and toggle switches.
Another kind of block is a wiring block. With a 13x20 layout, it’s advisable to provide multiple power lines from the transformer to different areas of the layout. I run DCS, so if this were my layout, the DCS gurus would suggest dividing it into 5-6 blocks where the center rails would be insulated between blocks. Legacy is similar in that fixed power goes to the track and the engines control the power to the wheels. I don’t know if Legacy suggest blocks or not, but it never hurts to insure there is adequate power all around the layout.
