In theory, there is no maximum amount of data that yEd can handle. In reality, it depends on the machine yEd runs on (available RAM, processor speed, etc) as well as the diagram structure and the chosen visual styles of diagram elements.
yEd is optimized for small to mid-sized graphs (i.e. several hundred elements). However, it is possible to work with diagrams that consists of a (few) thousands of elements as well - even though there might be a noticable slow down.