You are right that there should be no limit to the number of mesh cells, however, one of the key things about HEC-RAS 5's mesh is that it captures volume and cell face shape, so it shouldn't be necessary to have such small cells. As long as the cells capture the important aspects of your topography (ridges, especially), and are sufficiently small to get the information you need about the flow field, you should be able to increase the cells size a lot, compared to other 2D models, and still have a good model.