It is very variable, it depends on the model and the species.

To give an average, this is between 500,000 and 3,000,000 face.

For amazing renders, use an HDRI to create a nice ambience (you can find some for free at HDRI haven).

In Eevee, you can’t generate shadows with an HDRI, so use a Sun light instead, and adjust its intensity to your liking.

For clear shadows on the ground, use a low angle (between 0 and 5°).

In the sun settings, don't forget to activate "ground contact".

In the Eevee settings, also activate Ambient Occlusion and Bloom for a more realistic look.

Also makes sure to activate Screen Space Reflections.

If you want to take your realism even further, add a "volume irradiance" and bake the indirect lighting.

For the final render, don't forget to add a bit of depth of field for background blur. Also, use Filmic Color in Color Management for more realism.

And there you go! You’ll have an amazing scene in Eevee using the Vegetation addon.

Enjoy!

It's normal ! To optimize Blender's viewport, we replaced trees with bounding boxes. So you can navigate Blender easily. When rendering the trees will appear.

it's normal ! increase the transparency value in cycles.

(between 50 and 250 depending on your use)

The zip file about 3.6Go

After decompression 10Go