That's like the second most wasteful way of washing dishes, with the most wasteful being the same thing but not turning the tap off while scrubbing.
Well, presuming you have enough wares for a full wash. Filling the sink for just one plate would be unnecessary...
You can plug the sink and wash with your current tap method and see how many dishes it takes to fill the sink with water - that's how many you need to collect to save water with the sink method.