It's depends of the quality of the model. You can create 100.000 square mt with 1000 house (house like a box) and use 16 GB RAM
Or you can create 10 house and eat 24 GB.
From my test, as a "feelings", I think Corona eats more RAM than VRay, maybe because VRay bucket system.
But don't worry, you can crate good environment in Corona! ;-)
Use proxy, instances models, not big textures (if you don't need), CoronaScatter