The number of zone valves you put on a transformer depends on the situation.
A load is the user of electricity. Loads come in many familiar forms such as light bulbs, motors and doorbells. A load changes electrical energy into another form of energy, such as light, heat, sound, motion and magnetism.
The heart of a load is almost always a coil of wire of some sort. Electricity travels rather effortlessly along a straight wire. But when the wire is coiled, electricity has to work to get through. It's a little like when we take a walk. Moving along a sidewalk doesn't take a lot of effort. But if we come to a twisty path going uphill, we're suddenly working a lot harder. In fact, like electricity, we can create a lot of heat (illustrated by sweat) when we work hard. Since a load is usually a coil, I like to think of it as a spiral staircase for electricity. Running up a staircase creates heat.