It is very difficult to estimate those values, because is not as simple as multiply the size of the tree for the number of items. Also that depends also of the render engine. We suggest not use more than 4 to 5 millions of items with VRay and 500k for Mental Ray, but only for efficiency reasons. Over these limits, the scattering and render times would grow exponentially.
Anyway, with 5 millions of items, probably you will get more plants that pixels at screen... and it is not needed much memory to get this density (probably about 3Gb using a unique plant model of 1 Million of faces).